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/Exceptions/UserAlreadyInvitedException.php

@@ -0,0 +1,10 @@
+<?php
+
+namespace App\Exceptions;
+
+use Exception;
+
+class UserAlreadyInvitedException extends Exception
+{
+    //
+}