Milestone M34 - better GitHub integration, Plausible and more bug fixes

Let's go through what we got done in Milestone M34 (Apr 07 - Apr 20).

Enhancements:

• We upgraded GitHub integration to have better handling and communication back to the admin when a GitHub app connected to a course is disconnected from GitHub.

• We added Plausible integration on course pages for Bigbinary Academy to track relevant metrics.

Bug fixes:

• We fixed a few more ActiveRecord::InvalidForeignKey errors during incineration introduced into the app when we added all missing foreign keys as part of active_record_doctor audit.