[Notifications] A way to send test email

## Is your feature request related to a problem? Please describe.
When setting up Email notifications there is no way to verify the settings

## Describe the solution you'd like
Either an `ocis notifications command` or web interface button (or both) to trigger a test email.

## Additional context
Currently the only way is to create a share (or share a space) with another user and check the log for error messages.