[T.B.R.] Change signup

[SubSide]

As a user I want to be able to change my own signups

Acc 1: Able to change both its own signups as well as their intro(s)
Acc 2: This should be blocked if the mensa is already closed

TODO: Think about what should happen if there are multiple intros linked to an account (through an admin or something), but that person is a simple user. Do we need to make an exemption to 'submit' multiple intros? (As long as they're not adding more I guess?)

[Kipjr]

currently this results in the following error:

#1 /var/www/html/app/Http/Controllers/Api/v1/Mensa/Controllers/SignupController.php(112): abort()
#2 /var/www/html/vendor/laravel/framework/src/Illuminate/Routing/Controller.php(54): App\Http\Controllers\Api\v1\Mensa\Controllers\SignupController->signup()
#3 /var/www/html/vendor/laravel/framework/src/Illuminate/Routing/ControllerDispatcher.php(43): Illuminate\Routing\Controller->callAction()