Closes #1120: Grader should go to submissions

 - add of the rule for the grader to see the sub_menu in submissions
and results pages.
 - modification of tests which were, for me, obsolete.
Start work on API wrapper
Add user creation to the API wrapper
Remove User class, simpler to just manipulate hashes
Merge pull request #1122 from GhiGt/issue-1120

Closes #1120: grader should go to submissions
Merge pull request #1139 from danielstjules/issue-1125

Closes #1125: REST API wrapper for MarkUs