Closes #1074 : UI - Post deadline submissions

addition of a warning in the results/edit page for a post deadline
submission.
Checking in the controller instead of the view

Changing class 'error' with 'warning'
Another bug I forgot
Changing location of the method past_due_date? in model Grouping
Add unit tests

(problem with commit)
add teardown with destroy_repos to do not have the error of filenames
Merge pull request #1076 from GhiGt/issue-1074

Closes #1074 : UI - Post deadline submissions