Revert "Remove now unused mailer.boxUpdateAvailable()"

This reverts commit 558093eab1.

src/mail_templates/box_update_available.ejs

@@ -0,0 +1,55 @@
+<%if (format === 'text') { %>
+
+Dear <%= cloudronName %> Admin,
+
+Version <%= newBoxVersion %> for Cloudron <%= fqdn %> is now available!
+
+Your Cloudron will update automatically tonight. Alternately, update immediately at <%= webadminUrl %>.
+
+Changelog:
+<% for (var i = 0; i < changelog.length; i++) { %>
+  * <%- changelog[i] %>
+<% } %>
+
+Thank you,
+your Cloudron
+
+<% } else { %>
+
+<center>
+
+<img src="<%= cloudronAvatarUrl %>" width="128px" height="128px"/>
+
+<h3>Dear <%= cloudronName %> Admin,</h3>
+
+<div style="width: 650px; text-align: left;">
+    <p>
+        Version <b><%= newBoxVersion %></b> for Cloudron <%= fqdn %> is now available!
+    </p>
+
+    <p>
+        Your Cloudron will update automatically tonight.<br/>
+        Alternately, update immediately <a href="<%= webadminUrl %>">here</a>.
+    </p>
+
+    <h5>Changelog:</h5>
+    <ul>
+    <% for (var i = 0; i < changelogHTML.length; i++) { %>
+        <li><%- changelogHTML[i] %></li>
+    <% } %>
+    </ul>
+
+    <br/>
+    <br/>
+</div>
+
+<div style="font-size: 10px; color: #333333; background: #ffffff;">
+    Powered by <a href="https://cloudron.io">Cloudron</a>.
+</div>
+
+</center>
+
+<img src="https://analytics.cloudron.io/piwik.php?idsite=2&rec=1&e_c=CloudronEmail&e_a=update" style="border:0" alt="" />
+
+<% } %>
+