Add actionable context information to Email logs

In https://github.com/gocodebox/lifterlms/blob/master/includes/class.llms.engagements.php#L191, https://github.com/gocodebox/lifterlms/blob/master/includes/class.llms.engagements.php#L216 & https://github.com/gocodebox/lifterlms/blob/master/includes/class.llms.engagements.php#L220, there is no context information.

It doesn't help identify actionable information like:

1. Student,
2. Name of the Engagement email which in turn helps identify
1. Type of engagement
2. Lesson/Quiz/Assignment, etc
3. Course

Adding such information can help administrators, especially in case of failures and explore what action to take in such a case.