Edit connect.php, disconnect.php and 403.php to use Silex (experimental use)

Also rationalized usage of 403.php to "Forbidden" access (insufficient credentials) instead of "Unauthorized" access (401 HTTP Status Code : authentication is required).
In case of needed authentication : redirect to /connexion.php with parameter 'redirect_to' set to current request URI. The user will be redirected to this address on successfull authentication.
Fix credential issue in fimu/index.php for list route access.