Email log settings don't take effect until server is restarted

Other items in the Server Configuration admin page are generally expected to take effect immediately, but changes to the "Email log" settings (eg enabling log emails) only take effect at server startup.

This should be changed to be immediate, or failing that we should at least mention on the configuration page that a server restart is required.

(Note to QA: It might be a good idea to check if any other settings have similar bugs.)