Set may be missing some available help channels after initialisation
Sentry issue: [BOT-Z1](https://sentry.io/organizations/python-discord/issues/2354932564/?referrer=github_integration)

If the cog is reloaded while there are less than the maximum amount of available channels, it makes some channels available until the limit is reached. When a channel is made available, it updates the `available_help_channels` set. The `update_available_help_channels()` function will not update this set if it sees that the set already contains elements. This results in only the channels that were just made available being in the set; the set will not contain the channels that were already available when the bot started.

```
KeyError: <TextChannel id=776184670794678303 name='help-mushroom' position=268 nsfw=False news=False category_id=696958401460043776>
(1 additional frame(s) were not displayed)
...
  File "bot/exts/help_channels/_cog.py", line 469, in on_message
    await self.claim_channel(message)
  File "bot/utils/lock.py", line 108, in wrapper
    log.debug(
  File "bot/utils/lock.py", line 108, in wrapper
    log.debug(
  File "bot/utils/lock.py", line 108, in wrapper
    log.debug(
  File "bot/exts/help_channels/_cog.py", line 150, in claim_channel
    self.available_help_channels.remove(message.channel)

Unhandled exception in on_message.
```
diff --git a/bot/exts/help_channels/_cog.py b/bot/exts/help_channels/_cog.py
index afaf9b0b..cfc9cf47 100644
--- a/bot/exts/help_channels/_cog.py
+++ b/bot/exts/help_channels/_cog.py
@@ -267,6 +267,8 @@ class HelpChannels(commands.Cog):
             for channel in channels[:abs(missing)]:
                 await self.unclaim_channel(channel, closed_on=_channel.ClosingReason.CLEANUP)
 
+        self.available_help_channels = set(_channel.get_category_channels(self.available_category))
+
         # Getting channels that need to be included in the dynamic message.
         await self.update_available_help_channels()
         log.trace("Dynamic available help message updated.")
@@ -387,13 +389,10 @@ class HelpChannels(commands.Cog):
         )
 
         log.trace(f"Sending dormant message for #{channel} ({channel.id}).")
-        dormant_category = await channel_utils.try_get_channel(constants.Categories.help_dormant)
-        available_category = await channel_utils.try_get_channel(constants.Categories.help_available)
         embed = discord.Embed(
             description=_message.DORMANT_MSG.format(
-                dormant=dormant_category.name,
-                available=available_category.name,
-                asking_guide=_message.ASKING_GUIDE_URL
+                dormant=self.dormant_category.name,
+                available=self.available_category.name,
             )
         )
         await channel.send(embed=embed)
@@ -519,11 +518,6 @@ class HelpChannels(commands.Cog):
 
     async def update_available_help_channels(self) -> None:
         """Updates the dynamic message within #how-to-get-help for available help channels."""
-        if not self.available_help_channels:
-            self.available_help_channels = set(
-                c for c in self.available_category.channels if not _channel.is_excluded_channel(c)
-            )
-
         available_channels = AVAILABLE_HELP_CHANNELS.format(
             available=", ".join(
                 c.mention for c in sorted(self.available_help_channels, key=attrgetter("position"))
diff --git a/bot/exts/help_channels/_message.py b/bot/exts/help_channels/_message.py
index cf070be8..077b20b4 100644
--- a/bot/exts/help_channels/_message.py
+++ b/bot/exts/help_channels/_message.py
@@ -29,15 +29,15 @@ AVAILABLE_TITLE = "Available help channel"
 
 AVAILABLE_FOOTER = "Closes after a period of inactivity, or when you send !close."
 
-DORMANT_MSG = """
-This help channel has been marked as **dormant**, and has been moved into the **{dormant}** \\
+DORMANT_MSG = f"""
+This help channel has been marked as **dormant**, and has been moved into the **{{dormant}}** \\
 category at the bottom of the channel list. It is no longer possible to send messages in this \\
 channel until it becomes available again.
 
 If your question wasn't answered yet, you can claim a new help channel from the \\
-**{available}** category by simply asking your question again. Consider rephrasing the \\
+**{{available}}** category by simply asking your question again. Consider rephrasing the \\
 question to maximize your chance of getting a good answer. If you're not sure how, have a look \\
-through our guide for **[asking a good question]({asking_guide})**.
+through our guide for **[asking a good question]({ASKING_GUIDE_URL})**.
 """
 
 
['bot/exts/help_channels/_message.py', 'bot/exts/help_channels/_cog.py']