Add course (post) title as an email template and certificate merge code

**Is your feature request related to a problem? Please describe.**
When creating an engagement where the engagement type is sending an email, there's no merge tags to put things like the "related post" title into the email template. This means a separate template needs to be made for each course.

Similarly with certificates, it would be ideal if the same certificate template could be used across multiple courses.

**Describe the solution you'd like**
Merge code to insert the title of the related post ID that triggered the email. Also a merge code for the course title in a certificate (though this needs a check that there is one, as a certificate could be generated other ways)