Add a capability to control which users get emailed when a certificate is issued

[mattporritt]

Previously all users with the manage capability would get the email, now there is a separate email capability.
This is useful when there are sitewide roles that need to manage certificates at course level, but these users don't want/need to get emails everytime a certificate is generated

[mdjnelson]

This makes sense to me. Sorry for taking so long to get around to reviewing this. I have been focusing my efforts on https://github.com/markn86/moodle-mod_customcert which is a replacement module (please use that instead :)). I usually only touch this code now when a bug needs fixing and generally ignore improvements. However, this is simple enough and when I get time I will merge and backport.