Philipp&Joerg -
could you elaborate on:
i) First adapt the database model to the new features (article history, new user groups).
Currently user groups are not defined in the database, but in includes/DefaultSettings.php through the $wgGroupPermissions array. The _membership_ in particular groups is defined in the user_groups table. What need is there to change the database?
Brion was not on the list yet, so I just added him.