Folders

This PR brings in further integration for OAE in the following aspects:
 - Activities
 - Search
 - Previews
 - REST Docs
 - Extra functionality such as updating, deleting, ..
Rather than doing this filtering here, wouldn't it be better to check if the updated visibility is the same as the current visibility in the `ContentAPI.updateContentMetadata` function and just return straight away if it is? That would save a fair amount of queries as `ContentAPI.updateContentMetadata` will re-retrieve the content