Add approval status field to groups

[ross]

Super #1669

As groups need to be authorised in some cases, particularly publisher profiles, we need a field to denote how this is managed.

We should add an 'approval_status' field that will be used to denote its status. By default this will be "approved" but this will allow us to override it with a "pending" or "denied" status. The latter should also change the main status of the Group.