New frontend configuration mechanism.
Adds a generic frontend configuration mechanism, available at
/api/pleroma/frontend_configurations. This just takes a map and returns that as json, so we don't need to sync backend and frontend configuration names all the time.
It's open for any kind of frontends, right now there's a setting for 'pleroma_fe' but others can be added by the admin.
Closes #445 (closed).
Replaces !698 (closed).
Still needs the relevant frontend changes. No frontend changes needed.