Comment changes for federation

Comment changes for federation

This adds a new Comment link table that is used to link between some
object and then the comment object, which can be more or less any
object in Mediagoblin. The MediaComment has been renamed to
TextComment as that more aptly describes what it is. There is
migrations for these changes.

There is also the conslidation of the Report tables into a single
Report table, the same with the Notification objects. This is because
both of them split out MediaEntry and Comment versions into their own
polymorphic versions from a base, this is no longer a meaningful
distinction as comments can be anything.