Replace sentences by keys in translations file

[nicosomb]

See #1692 (comment)

Some good starting point:

• http://jolicode.com/blog/translation-workflow-with-symfony2
• http://developer.happyr.com/how-happyr-work-with-symfony-translations

[0wnrepo]

Give an example of how to do this, and I can pick it up on Tuesday. Or anybody else if it's faster.
ping @j0k3r

[nicosomb]

See Working with translation placeholder / It’s also a good practice to use abstract keys instead of plain string: here http://jolicode.com/blog/translation-workflow-with-symfony2

[j0k3r]

Yeah we just need to define a convention on how we are going to use keys. Like [BUNDLE].[MODULE].[LABEL] or sth else

[nicosomb]

Good job, @j0k3r!