Implement grades

[sirchnik]

It would be nice to have the optional feature of sending grades back to the LMS.
This requires #7
It makes sense that there should be a difference between ungraded and graded exercises which do not show the solution.
Implementation should be done with the LTI Assessment and Grading System.

Notes:

• DataModel images are being served by nginx so there is no access control in place.
• AGS has to be allowed in LMS.