Use scheduler for silencing
I was looking through the code of [`silencer.py`](https://github.com/python-discord/bot/blob/1d3acb0a222a885c5ef42e91570a00f91976fb5b/bot/cogs/moderation/silence.py) and I found this:

https://github.com/python-discord/bot/blob/1d3acb0a222a885c5ef42e91570a00f91976fb5b/bot/cogs/moderation/silence.py#L93

With this approach, in case the channel was unsilenced and re-silenced back, it might get unsilenced by this timer. This is a relatively rare occurrence as this would have to happen within the 15 minutes limit, but nevertheless it seems more logical to use `Scheduler` in here, which can be canceled when the channel is manually unsilenced, to prevent this.
diff --git a/bot/cogs/moderation/silence.py b/bot/cogs/moderation/silence.py
index 25febfa5..c8ab6443 100644
--- a/bot/cogs/moderation/silence.py
+++ b/bot/cogs/moderation/silence.py
@@ -1,7 +1,7 @@
 import asyncio
 import logging
 from contextlib import suppress
-from typing import Optional
+from typing import NamedTuple, Optional
 
 from discord import TextChannel
 from discord.ext import commands, tasks
@@ -11,10 +11,18 @@ from bot.bot import Bot
 from bot.constants import Channels, Emojis, Guild, MODERATION_ROLES, Roles
 from bot.converters import HushDurationConverter
 from bot.utils.checks import with_role_check
+from bot.utils.scheduling import Scheduler
 
 log = logging.getLogger(__name__)
 
 
+class TaskData(NamedTuple):
+    """Data for a scheduled task."""
+
+    delay: int
+    ctx: Context
+
+
 class SilenceNotifier(tasks.Loop):
     """Loop notifier for posting notices to `alert_channel` containing added channels."""
 
@@ -53,15 +61,25 @@ class SilenceNotifier(tasks.Loop):
             await self._alert_channel.send(f"<@&{Roles.moderators}> currently silenced channels: {channels_text}")
 
 
-class Silence(commands.Cog):
+class Silence(Scheduler, commands.Cog):
     """Commands for stopping channel messages for `verified` role in a channel."""
 
     def __init__(self, bot: Bot):
+        super().__init__()
         self.bot = bot
         self.muted_channels = set()
         self._get_instance_vars_task = self.bot.loop.create_task(self._get_instance_vars())
         self._get_instance_vars_event = asyncio.Event()
 
+    async def _scheduled_task(self, task: TaskData) -> None:
+        """Calls `self.unsilence` on expired silenced channel to unsilence it."""
+        await asyncio.sleep(task.delay)
+        log.info("Unsilencing channel after set delay.")
+
+        # Because `self.unsilence` explicitly cancels this scheduled task, it is shielded
+        # to avoid prematurely cancelling itself
+        await asyncio.shield(task.ctx.invoke(self.unsilence))
+
     async def _get_instance_vars(self) -> None:
         """Get instance variables after they're available to get from the guild."""
         await self.bot.wait_until_guild_available()
@@ -90,9 +108,13 @@ class Silence(commands.Cog):
             return
 
         await ctx.send(f"{Emojis.check_mark} silenced current channel for {duration} minute(s).")
-        await asyncio.sleep(duration*60)
-        log.info("Unsilencing channel after set delay.")
-        await ctx.invoke(self.unsilence)
+
+        task_data = TaskData(
+            delay=duration*60,
+            ctx=ctx
+        )
+
+        self.schedule_task(ctx.channel.id, task_data)
 
     @commands.command(aliases=("unhush",))
     async def unsilence(self, ctx: Context) -> None:
@@ -103,7 +125,9 @@ class Silence(commands.Cog):
         """
         await self._get_instance_vars_event.wait()
         log.debug(f"Unsilencing channel #{ctx.channel} from {ctx.author}'s command.")
-        if await self._unsilence(ctx.channel):
+        if not await self._unsilence(ctx.channel):
+            await ctx.send(f"{Emojis.cross_mark} current channel was not silenced.")
+        else:
             await ctx.send(f"{Emojis.check_mark} unsilenced current channel.")
 
     async def _silence(self, channel: TextChannel, persistent: bool, duration: Optional[int]) -> bool:
@@ -140,6 +164,7 @@ class Silence(commands.Cog):
         if current_overwrite.send_messages is False:
             await channel.set_permissions(self._verified_role, **dict(current_overwrite, send_messages=None))
             log.info(f"Unsilenced channel #{channel} ({channel.id}).")
+            self.cancel_task(channel.id)
             self.notifier.remove_channel(channel)
             self.muted_channels.discard(channel)
             return True
diff --git a/tests/bot/cogs/moderation/test_silence.py b/tests/bot/cogs/moderation/test_silence.py
index 3fd149f0..ab3d0742 100644
--- a/tests/bot/cogs/moderation/test_silence.py
+++ b/tests/bot/cogs/moderation/test_silence.py
@@ -127,10 +127,20 @@ class SilenceTests(unittest.IsolatedAsyncioTestCase):
             self.ctx.reset_mock()
 
     async def test_unsilence_sent_correct_discord_message(self):
-        """Proper reply after a successful unsilence."""
-        with mock.patch.object(self.cog, "_unsilence", return_value=True):
-            await self.cog.unsilence.callback(self.cog, self.ctx)
-            self.ctx.send.assert_called_once_with(f"{Emojis.check_mark} unsilenced current channel.")
+        """Check if proper message was sent when unsilencing channel."""
+        test_cases = (
+            (True, f"{Emojis.check_mark} unsilenced current channel."),
+            (False, f"{Emojis.cross_mark} current channel was not silenced.")
+        )
+        for _unsilence_patch_return, result_message in test_cases:
+            with self.subTest(
+                starting_silenced_state=_unsilence_patch_return,
+                result_message=result_message
+            ):
+                with mock.patch.object(self.cog, "_unsilence", return_value=_unsilence_patch_return):
+                    await self.cog.unsilence.callback(self.cog, self.ctx)
+                    self.ctx.send.assert_called_once_with(result_message)
+            self.ctx.reset_mock()
 
     async def test_silence_private_for_false(self):
         """Permissions are not set and `False` is returned in an already silenced channel."""
['tests/bot/cogs/moderation/test_silence.py', 'bot/cogs/moderation/silence.py']