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'm not quite sure I agree with this. It looks like instance is hardly used in the code that exercises comments, and the amount of code introduced to assign instance to this model might be greater than the code necessary to resolve instance through contenttypes (which is probably easier than you think). What tips the scales for me is the whole extra database column.