notifications: send email when manual platform update is required

src/mail_templates/box_manual_update_required-text.ejs

@@ -0,0 +1,14 @@
+Dear <%= cloudronName %> Admin,
+
+Cloudron v<%= version %> is available.
+
+Changelog:
+<%- changelog %>
+
+Go to the Updates view to update: <%= updateUrl %>
+
+Powered by https://cloudron.io
+
+Don't want such mails? Change your notification preferences at <%= notificationsUrl %>
+
+Sent at: <%= new Date().toUTCString() %>