Run preupdate before an update

The update routine tries to test the database migration before actually performing the update.

However, this will fail hard if the schema has changed (for example an unique key has been added). App developers can convert the DB in preupdate.php, however it is not called before and therefore the update fails.

This actually breaks ownCloud updates from ownCloud 6 to ownCloud 7 when the files_antivirus app is enabled.
Merge pull request #12109 from owncloud/add-preupdate-before-upgrade

Run preupdate before an update
Merge branch 'stable7-upstream' into stable7-rotdrop
fix for issue #10880
removal of wrong/double implemented check

Check already implemented in core/settings/ajax/changedisplayname.php
cleanup group admin(s) on deleteGroup
Hiding add to your own cloud if server2server sharing is not enabled
Merge branch 'stable7-upstream' into stable7-rotdrop