Enforce signup priority correctness

[georgelgeback]

Basically this changes:

• We no longer allow people to create or edit signups with priorities they should not be allowed to use. Previously a regular member could enter "Ordförande" as their priority even if it was not supported for the event, and if they weren't actually ordförande.
• We now check that a user has the nollning priority they claim, previously a mentee could claim to be a mentor, or vice-versa.
• GetMyPriorities now returns all your priorities, not just the most recent one. This probably should not matter but in the rare case someone is both a mentee and mentor, or mentor for both mission and mentor groups, this should return both their roles.

[hannesblaman]

Realized that group system doesn't care about which year's nollning your group belongs to. Fixed accordingly.

Otherwise LGTM!

[hannesblaman]

On second thought, I reworked the get_user_nollning_priorities funtction to only loop through the groups that the user actually has. Should optimize with less unnecessary looping but keep the same functionality.

If you are happy with that change then it LGTM. Re-approved!