Add view for retrieving comments for moderation

Switches to using a custom comments app, which is a thin wrapper around
ThreadedComments with a few extra fields for moderation.

Flagging comments will be implemented in a later PR
I actually tried going through the content type system first.
You can't use the Django ORM to select related fields of your `GenericForeignKey` fields, you have to do a subquery on the `object_pk` field.  Doing this led to cleaner code in the view function.