Re-enabled plaintext view for email notifications

Updated mail notifications to set the HTML and plaintext views since before
no plaintext version was being created.

Closes #1182

app/Notifications/MailNotification.php

@@ -0,0 +1,35 @@
+<?php namespace BookStack\Notifications;
+
+use Illuminate\Bus\Queueable;
+use Illuminate\Contracts\Queue\ShouldQueue;
+use Illuminate\Notifications\Messages\MailMessage;
+use Illuminate\Notifications\Notification;
+
+class MailNotification extends Notification implements ShouldQueue
+{
+    use Queueable;
+
+    /**
+     * Get the notification's channels.
+     *
+     * @param  mixed  $notifiable
+     * @return array|string
+     */
+    public function via($notifiable)
+    {
+        return ['mail'];
+    }
+
+    /**
+     * Create a new mail message.
+     * @return MailMessage
+     */
+    protected function newMailMessage()
+    {
+        return (new MailMessage)->view([
+            'html' => 'vendor.notifications.email',
+            'text' => 'vendor.notifications.email-plain'
+        ]);
+    }
+
+}