Add Devise based authentication for RailsAdmin

* pull out RailsAdmin conf from models classes to an initializer
* the model User has a boolean `admin` field
* add God admin account in seeds
* users can't sign up by themselves
* cleaned up devise views markup and content (in french)

https://trello.com/c/FgCacpo9
I don't know what to do here. We've started writing everything in english, with an i18n branch with work on translations. But I don't know if it would be a lot of work to keep this emails in english, and use i18n to translate them ?

(Yep. I hop this little app to be used in other countries ;))