Add file repo at setup time, not on registration
Adding a repo to $wgForeignFileRepos relies on the defines BS_DATA_DIR
and BS_DATA_PATH. They are provided by BlueSpiceFoundation however the
extension might not have been loaded yet. For example:
wfLoadExtensions( [ 'BlueSpiceAvatars', 'BlueSpiceFoundation', ] );
In this case the defines are not set and PHP notices are sent.
Move the configuration to the SetupAfterCache.
Setup.php automatically sets the 'backend', but that is done before the
SetupAfterCache hook. Add a backend named Avatars-backend.