Closes #265: Adding possibility to map grader to section

 - addition of a select in graders/index
 - modification of the filters tables, addition of a rule
added i18n for section
added ':' to section's name
Closes #1133: Security flaw in download action

- edited authorized_to_download? method in ResultController
Merge pull request #1137 from GhiGt/issue-1133

Closes #1133: Security flaw in download action
Merge pull request #1128 from GhiGt/issue-265

Closes #265: adding possibility to map grader to section