Add Mastodon API service class, auth token config

Add a new REST API service class to keep things DRY

app/views/admin/settings/services/_mastodon.html.erb

@@ -16,5 +16,10 @@
           key: :mastodon_address_domain,
           title: "User address domain"
         ) %>
+    <%= render FormElements::FieldsetResettableSettingComponent.new(
+          key: :mastodon_auth_token,
+          type: :password,
+          title: "API auth token"
+        ) %>
   <% end %>
 </ul>