Prevent creating groups after a deadline

[LittleHelicase]

It is rather complicated to correctly reassign solutions if the group members change. Thus we currently want to disallow creating new groups after the first week. For this we need a group-deadline configuration option.

[leMaik]

That's something the server should do, but an API request like /api/groups/deadline would be useful to display a nice message on the client. Do we need to synchronize the time here?

[5cript]

Is this still an issue? because we now have a possibility to correctly assign solutions to users if they join another group.