Warn when student repo is behind the source assignments repo

[meling]

Autograder should be able to detect when a student repo or a group repo is behind the original assignments repo, and give a warning, along with appropriate git instructions (in the frontend) for merging and resolving any conflicts.

[JoachimTislov]

Refer to #1163, The simplest solution would be to create a pull request in all of the students repositories when a teacher pushes to the assignments repository.

We can also create a notification through the planned notification system as well.

[meling]

Once we've implemented #1409 this type of notification would become somewhat irrelevant, since GitHub would show that their repository is behind the assignments repository.