Add a switch to turn mail sending on/off

Currently it is not possible in Joomla to stop the site from sending e-mails. This is a nuisance if you restore a backup from your live site for testing purposes and the installation starts sending out e-mails regarding user subscriptions for example.

I am aware you can use bogus SMTP settings but that doesn't sit well with me as I am never sure that is enough. A clean switch to turn mail sending on/off has my preference.

This PR implements this feature.