✨(courses) add course synchronization on enrollment count and release 5.12.0

[sampaccoud]

Purpose

We have recently introduced enrollment counts in Richie. As a result, we now need to update the count each time an enrollment is made.

Proposal

• [x] Add "enrollment_count" to the payload sent to the synchronization hook
• [x] Add a signal to update the course run info each time an enrollment is modified

🔖 Release version to 5.12.0

Added

• Add enrollment count to the course run synchronization hook payload