Rename user to contributor in all relevant places

[kiragrammel]

In the prototype-pair-programming branch in the table submissions the column user_id and user_type were changed to contributor_id and contributor_type because a exercise can now also be submitted as a group.
The alias method alias_method :user, :contributor was added for this purpose in the ContributorCreation. It is cleaner if this is not needed and all places use the contributor instead.

Also, it probably makes sense to talk about contributor instead of user in the following other tables as well, allowing groups to be contributors as well:

• ❌ request_for_comments -> An individual user is requesting the RfC and others might have access through the group
• ✅ runner
• user_exercise_interventions
• ❌ user_proxy_exercise_exercises (relevant e.g. for Java courses) -> This assignment to a proxy exercise is performed before forming a group. Therefore, it is always an individual.
• ❌ user_exercise_feedbacks -> Individuals provide feedback; this could be different for all members of the group. Since we have the link to the submission, we can always reconstruct who was a member of the group (if any).