setup Paperclip with Amazon S3, add upload avatar tu user profile, display avatar on top bar and profile page
set default avatar with correct size, redefine flash messages (success, warning, alert, info), separate redirect after signed in for admin and user
root to landing page when user unauthenticated but root to profile when authenticated, display footer only when user unauthenticated (landing page, connection, inscription)
add devise.fr.yml for internationalization and improve translation