Closes #1078 : Be able to download all files submitted by a group

Addition of a method (get) 'downloads' to return a zip file of all
files in the repository.
 - Move from link to button.
 - Addition of translations.
 - Modification request from get to post.
Add button in result/edit page

- Add a conditional button in edit if there is files
- Add error checking in submission_controller.
Remove folders problems

 - Change directory's name of zipfile
Resolve buttons position issues

 - change type of method downloads in 'get'
 - (bonus) small indentation
First changes for the tests
Mergin with origin/master
Adding a test for 'downloads' method in SubmissonConstroller
Add test when downloads is incorrect
respect 80 chars.
Merge pull request #1084 from GhiGt/issue-1078

Closes #1078 : Be able to download all files submitted by a group