Post format support

[janboddez]

We could disable federation of certain post formats (i.e., allow only those formats selected on the settings page, if post formats are supported by the active theme).

It'd work the same as our local-only category. So we'd also have to filter outboxes and fix post counts.

Or one could just add the local-only category to these types, of course. Could probably even be done automatically (as long as it happens early enough in the "save/publish post" request to actually prevent federation!