Update RegisterController for PeriodicEvents API change

This was missed in #532 ("Introducing PeriodicEvents instead of
EventTriggers"). Attempting to open the "Application for college
students" tab therefore resulted in a 500 error code because of the
call to the undefined `getApplicationDeadline` method.

Author: BertalanD

app/Http/Controllers/Auth/RegisterController.php

@@ -50,7 +50,7 @@ public function showRegistrationForm()
     {
         return view('auth.register', [
             'user_type' => Role::COLLEGIST,
-            'application_open' => ApplicationController::getApplicationDeadline() > now(),
+            'application_open' => app(ApplicationController::class)->isActive(),
             // 'countries' => require base_path('countries.php'),
         ]);
     }