[Feedback] Summarize proposed changes in a merge request

Description

This issue is for providing feedback on the summarize merge request changes experiment that's available for merge requests.

Summaries in merge requests are now generated automatically after a commit and can be viewed in the following places:

To-Do list (available when someone requests a review from you)
Email notifications (available when someone requests a review from you)
Merge request description (available in the MR body before the description)
Merge request sidebar (available on the MR page, on the top right action menu (kebab menu icon))

Previously, the feature was enabled via a /summarize_diff quick action, but this will be deprecated and removed in the future.