mailer: add html version of test mail

src/mail_templates/test-html.ejs

@@ -0,0 +1,20 @@
+<center>
+
+<p>Test email from  <%= cloudronName %>.</p>
+
+<br/>
+
+<p>If you can read this, your Cloudron email settings are good.</p>
+
+<br/>
+
+<p>Sent at: <%= new Date().toUTCString() %></p>
+
+<br/>
+<br/>
+
+<div style="font-size: 10px; color: #333333; background: #ffffff;">
+  Powered by <a href="https://cloudron.io">Cloudron</a>
+</div>
+
+</center>

src/mail_templates/test.ejs

@@ -4,6 +4,8 @@ Test email from <%= cloudronName %>,
 
 If you can read this, your Cloudron email settings are good.
 
+Powered by https://cloudron.io
+
 Sent at: <%= new Date().toUTCString() %>
 
 <% } else { %>