Refactor join_mobile view with info about Mconf-Mobile, refs #807

Partial cherry-pick of 9ff501adfc.
Use new locale to display success message when changing languages

Partial cherry-pick of bbda416adfd.
Refactoring private messages

Mostly a refactor in the index view, but a little in other views as well.
There are still things to be improved.
Show translated string when there are no attachments in the doc tab

Partial cherry-pick of 79724860dac.
Use correct titles for tags in attachments
Always point to space/show in the list of spaces

No need to point directly to join_requests/new, it should be done automatically
if the user tries to access a space he cannot access.

Same as was done in the branch master by 4603e33fbbe38.