New certificate notification and engagement

[brianhogg]

Description

Adds new Email notification for Certificate Earned, and new Engagement for "student earned a certificate"

Also allows the "related post ID" to be optional, so an engagement can apply to any Certificate, Course, etc.

TODO:

• Improved link that lets them login if they're not (and sharing isn't enabled) and view the certificate immediately, vs. the current 404 error. Will be handled by Redirect Certificate Links to Login if Private #3050
• Add certificate link to the default notification email template (should be there with the footer, but email notification not showing it?)
• Add merge code for certificate link for engagement

How has this been tested?

Manually

Screenshots

Checklist:

• This PR requires and contains at least one changelog file.
• My code has been tested.
• My code passes all existing automated tests.
• My code follows the LifterLMS Coding & Documentation Standards.