Folders

This PR brings in further integration for OAE in the following aspects:
 - Activities
 - Search
 - Previews
 - REST Docs
 - Extra functionality such as updating, deleting, ..
I think so, it allows for a cleaner separation with the existing content previews. Adding another queue to rabbitmq isn't really expensive.

At some point we should refactor how content previews are generated (get rid of the REST layer). Not having the folders mixed in might make that work easier.