Add support for user groups

[nbars]

• Each user can/must choose a group they want to participate in during registration.
• For scoring, we probably can use the best submission per group.
• What is about grading? Currently, grading is streamlined for single users. Also, which submission from which user would be the one to grade?

[nbars]

The easiest way would probably be to make all users members of some group (even if the size is 1) by default. Scoring and grading would happen on a per-group basis.