PROJ-92 Registration views revision

- Updated the registration views, (removed useless links and added some bootstrap styling)
- Translated a couple of registration views
- Added AJAX form submission and validation, but without any example for now
- Added field validation for User model
- Added internationalization (fr.yml, devise.fr.yml), with User model attributes
- Added gems: "Simple Form", "Client Side Validations", "Jasny Bootstrap" and internationalization (rails-i18n)