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.