Closes #570 : Assignment due date with sections

- Add error message to show which section(s) is in past
- Add method to know the sections' names
- Modification of past_due_date? which doesn't work in some cases
Add tests for the new assignment method
Closes #860 : Sort assignments in dashboard

 - Adding a order of assignments in the MainController
 - Adding a test
Merge pull request #1082 from GhiGt/Closes-#570

Closes #570 : Assignment due date with sections
Merge branch 'PR-862'
Merge branch 'master' of github.com:benjaminvialle/Markus into machinist2