Add optional prefix to GUI routes

[bynicolas]

Makes a config option to add a prefix to GUI routes. If we only override the default routes to add a prefix, we get a LogicException when using php artisan route:cache .

The config option is set in the .env file using ROLES_GUI_ROUTES_PREFIX.

   LogicException

  Unable to prepare route [prefix/roles] for serialization. Another route has already been assigned name [laravelroles::roles.index].

[jeremykenedy]

Thank you!