Add associated post's merge codes to certificates (and emails, achievements?)

Adding the information from the associated post (quiz, lesson, course, etc) can allow certificates, emails and achievements to behave like templates that can be used across courses instead of the current situation where you have to create a separate certificate for all courses.

This will save a lot of database entries and make things easier in case the site has a lot of courses (I have encountered sites with 200 or so courses where site owners have had to create exactly identical certificates for each course only to have a different course title).

I do understand that this could be a little complicated because at the level of the certificate, you'd have to know what engagements are they attached to and the merge codes that can be available accordingly. However, that can be simplified by using generic merge codes for the associated post like `{trigger_post_title}` which can stand for the course, quiz or lesson.

@thomasplevy what do you think?