Rewrite event priority and user types

We need to make the user types more robust. Right now they are decided by checking which groups the user are in which is prone to error. The user can for example be both a novice and mentor at the same time which is not very logical.

Start by locating the event-priority logic and the group-based user-type checks in the Ruby on Rails application. Review how users can currently receive multiple types, then clarify the intended mutually exclusive rules with maintainers. Done should include the rewritten classification and tests covering conflicting group memberships.

Written by the indexing model from the issue text.