Added services

This commit moves most controller logic onto Services. Services are part of the Service-Repository pattern. The models act as repositories.

Services are easily testable and are needed for the upcoming API, in order to avoid duplicated code and to maintain a single source of "truth".

 The User, Vacancy and Vote controllers still need their logic moved onto services.

app/Http/Controllers/HomeController.php

@@ -25,6 +25,8 @@ use App\Vacancy;
 
 class HomeController extends Controller
 {
+    // doesn't need a service, because it doesn't contain major logic.
+
     /**
      * Show the application dashboard.
      *