Add option for deletion warnings

[bfriesenvcc]

We stopped archiving and instead keep all of our courses for 2 years and then delete them. We wanted to give our instructors warning of the upcoming deletion, so I made changes to the language pack so that I could continue to use the great features of this plugin, most importantly the email notification.

I think having this option added so that both archiving and deletion could be accommodated without having to make these changes would be a bonus.

In the meantime, it would be helpful if there was an option to EXCLUDE the opt out option from the outgoing email. This would eliminate the need to change the course_skipped and optoutarchive strings shown in the attached image. I will create another issue for that. :-)