notifications: add back app down and app oom mails

src/mail_templates/app_down.ejs

@@ -0,0 +1,23 @@
+<%if (format === 'text') { %>
+
+Dear Cloudron Admin,
+
+The application '<%= title %>' installed at <%= appFqdn %> is not responding.
+
+This is most likely a problem in the application.
+
+To resolve this, you can try the following:
+
+* Restart the app by opening the app's web terminal - https://docs.cloudron.io/apps/#web-terminal
+* Restore the app to the latest backup - https://docs.cloudron.io/backups/#restoring-an-app
+* Contact us in our Forum at https://forum.cloudron.io
+
+Powered by https://cloudron.io
+
+Don't want such mails? Change your notification preferences at <%= notificationsUrl %>
+
+Sent at: <%= new Date().toUTCString() %>
+
+<% } else { %>
+
+<% } %>