During documentation of the pair programming feature, we noticed that the Contributor should be an abstract class, which is parent for the User and the ProgrammingGroup. With this commit, we perform these changes.
So far, a programming group is always considered to be a learner. While we could, technically, consider it to be a teacher or an admin if all members have this role, this behaviour might have unintended side effects.
* Correct sorting in table
* Modify page when nested in exercises
* Fix links between pages
* Link from statistics page to programming_groups/index
* Link from submission page to programming_groups/<id>
* Allow filtering for exercise ID on ProgrammingGroup#index
* Add search fields for internal and external user id on pg/index
* We want to identify a user that has triggered a testrun. Previously (in regular operation), only submission author who were regular users were able to start a testrun. Now, we want to prepare a future where submission authors are programming groups. Still, a testrun is triggered by an individual user and not a group.
* Further, this commit fixes some missing foreign key constrains.
