Root size for home storage is now size of "files" subdir

Fixes #4593 

This change should make it work as discussed: only files under "/files" will count in the total size and in the quota.
Metadata and app data, including trashbin are outside of it.

Please review @karlitschek @schiesbn @icewind1991 @DeepDiver1975 

@schiesbn not sure whether further changes are needed in trashbin/versions to make them work correctly with this change ?