Move API to use JSON:API standards and fractal serializer

Makes the data slightly more complex, but forces a standard and can
always be changed down the road simply by changing the default
serializer.

app/Transformers/User/SubuserTransformer.php

@@ -38,6 +38,7 @@ class SubuserTransformer extends TransformerAbstract
     public function transform(Subuser $subuser)
     {
         return [
+            'id' => $subuser->id,
             'username' => $subuser->user->username,
             'email' => $subuser->user->email,
             '2fa' => (bool) $subuser->user->use_totp,