Switch from auth tags to UserMan's group feature

[s-ludwig]

• Transition from string tag based authentication groups to UserMan groups
• Add a group management UI for adding/deleting groups and managing members (with the latest UserMan version, this would also be possible using its own web admin interface as a temporary solution)
• Add a DB upgrade procedure to convert existing tags to groups

See #32.

[streaps]

auth tags are still broken?

[s-ludwig]

I just tested it and they are indeed still broken, because the auth groups are not created. This requires an addition to the admin UI to be able to add/remove groups and change their description.

It would then also make sense to improve the UI for assigning auth groups to users and news groups using a drop down list instead of the current text based entry.