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 can revert back devise views to the default ones and fix only markup. Good for you?