Now when a user tries to access the route to send a new join request, and he is already a member, he will be redirect to the space home page.

cherry-pick caf4656865b
Add a flash msg redirecting from join_requests/new to space/show, refs #817

Complements 5b2d2603.
Add translations to user roles in spaces

Partial cherry-pick of 8e5e2915e80 and 3bf701a3e06.