Added restriction tests and fixed any bugs in the process

Also updated many styles within areas affected by the new permission and roles system.

app/Exceptions/NotFoundException.php

@@ -0,0 +1,14 @@
+<?php namespace BookStack\Exceptions;
+
+
+class NotFoundException extends PrettyException {
+
+    /**
+     * NotFoundException constructor.
+     * @param string $message
+     */
+    public function __construct($message = 'Item not found')
+    {
+        parent::__construct($message, 404);
+    }
+}