Adds feature to permit e.g., local admins and community moderators to automatically follow all newly registered accounts

In practice, it was already removed half a year ago, but the description
 and cheatsheet entries were still there.

The migration intentionally does not use ConfigDB.get_by_params, since
this will break migration code as soon as we add a new field is added
 to ConfigDB.

Closes #2086

changed namespace for activity expiration configuration

This reverts commit 72155636.

Rich media improvements

See merge request !2944

not needed options deletion

Use a separate pool for preview requests