Group revamp

[Víctor López Juan]

Groups have a lot of issues:

• https://bitbucket.org/cse-fire/fire/issues/51/student-cannot-resubmit-after-changing-the
• https://bitbucket.org/cse-fire/fire/issues/113/add-group-number-to-exported-student
• https://bitbucket.org/cse-fire/fire/issues/71/group-join-codes-may-jeopardize
• https://bitbucket.org/cse-fire/fire/issues/56/retroactively-add-a-submitter-to-a
• https://bitbucket.org/cse-fire/fire/issues/25/more-clear-procedure-for-people
• https://bitbucket.org/cse-fire/fire/issues/108/leading-0-in-group-number

See wiki for details of one possible solution derived from the two meetings we had with @aztek and Fredrik Lindblad, and some further discussion with @bafain after the second meeting.

Add other possible solutions you can think of after the existing ones.