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 agree that it's a bit of an unfortunate name but it is following the implicit naming convention similar to `FoldersAPI`, `FoldersDAO`, ..

It makes sense in my head because we're requiring the `folders` file from the `library` directory in `oae-folders` and it deals with listing `folders` (as opposed to `content ` for which we have a `FoldersContentLibrary`.