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.
Tags can be added to exercises in the edit view. Tags can monitored under /tags.
Added the concept of ProxyExercises which are a collection of Exercises. They can be found under /proxy_exercises
Added Interventions as prework to show interventions later to the user.
Added exercise/[:id]/working_time to return the working time of the user in this exercise and the average working time of all users in this exercise
