Folders

This PR brings in further integration for OAE in the following aspects:
 - Activities
 - Search
 - Previews
 - REST Docs
 - Extra functionality such as updating, deleting, ..
We might be able to give this a more descriptive name (e.g. `_getFoldersForContent`). It also feels like this function should be exposed from elsewhere (e.g. `authz.js`).