Toggle notification via sidebar control

[anoved]

I may only want email notifications for unattended or particularly long-running print jobs. Therefore I want to enable email notifications on a job-by-job basis. (All other settings will remain the same.) Currently this is achieved by the Enabled checkbox in Email Notifier settings, which works fine (and requires user to visit Settings if they want to try it out, ensuring an opportunity to fill in other required settings). However it may be more streamlined to present that control in the main interface sidebar.