Enable ePUB plugin to run by default

### Observed behavior

ePUB plugin is not enabled by default, and it prevents from importing & viewing ePUB files, until the command `kolibri plugin kolibri.plugins.document_epub_render enable` is run.

### User-facing consequences
Inability to view and import ePUB files.


### Context
dev environment, tried on `develop` and `0.11.a7` branches

diff --git a/kolibri/utils/conf.py b/kolibri/utils/conf.py
index dd308d001b..0576a182b5 100644
--- a/kolibri/utils/conf.py
+++ b/kolibri/utils/conf.py
@@ -68,6 +68,7 @@ except ImportError:
         "kolibri.plugins.user",
         "kolibri_exercise_perseus_plugin",
         "kolibri.plugins.style_guide",
+        "kolibri.plugins.document_epub_render",
     ]
 
 #: Everything in this list is added to django.conf.settings.INSTALLED_APPS
['kolibri/utils/conf.py']