Closes #1073: Possibility to import/export assignments

 - add upload and download link
 - add actions in AssignmentController
 - modification of required fields for an assignment

/!\ Some modification are still needed
added encoding with iconv
added some tests and fixed a bug
Merge branch 'master' into issue-1073
somes small changes
Merge branch 'master' into issue-1073
Merge master
last changes

- fix bad i18n
Moving fixtures files

 - Removed test/classlist-csvs/ which was unused
 - Moved test/group_csvs/ into fixtures/
Student raw html

 - added `html_safe` method.
Merge pull request #1150 from GhiGt/student-raw-html

Student raw html
Merge pull request #1149 from GhiGt/moving-files

Moving fixtures files
fix bug
Merge pull request #1127 from GhiGt/issue-1073

Closes #1073: Possibility to import/export assignments
Require description on assignment creation
Merge pull request #1151 from danielstjules/issue-1126

Part of fix for Issue 1126: Require description on assignment creation