Do not track usage per group

[surt91]

Since users can have multiple groups, it is not clear to which group usage or ratings should be accounted to. For clarity, we should remove the group field from the usage tables.

Since the group usage is not shown in the dashboard, this is a rather technical issue.

This is clean-up of #316.

[RichardSiegel]

All Limits, per user and per group are unclear if there are multiple groups for a single user.
How do we want to handle that?

[surt91]

When discussing #316, we decided to remove the per-group limit.

The per-user limit was not considered yet. I would suggest to take the maximum quota of all groups a user has. The current implemenation is to use the "first" group (which is not well defined).