Features for course maintenance, one button to test all submissions with model solutions.

[jamo]

So that we can easily test that all model solutions are correct and that dependencies etc are met at the sandbox. (Running Xvfb etc)

[mpartel]

Implementation suggestion:

• Make submissions of all the model solutions. Make them owned e.g. by the 'test' user that we create in seeds.rb.
• Put all the submission IDs into a table that keeps track of which model solutions have been tested for the course.
• For bonus points: delete from that table or mark obsolete exercises that change during a course refresh, and prompt the admin to test those again.