notifications: no email for app up/down/oom events

emails will not be used for self monitoring events. these are best done
from the outside. we just log everything in eventlog and raise notifications
as well.

src/mail_templates/app_down.ejs

@@ -1,21 +0,0 @@
-<%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 via <%= supportEmail %> or https://forum.cloudron.io
-
-
-Powered by https://cloudron.io
-
-Sent at: <%= new Date().toUTCString() %>
-
-<% } else { %>
-
-<% } %>