Allow disabling emails via configuration options

At the moment GGTX would try to send emails on certain events (like when a corpus finishes importing).

This is fine for non-local servers, but we should be able to control the email settings via our settings.toml, so that we can disable sending emails altogether (note that in practice no email would be actually sent out, but you will still see the error in the logs, and it might have ripple effects on some other things, so we should just skip the step completely).