Load commit details async

What does this MR do and why?

Asynchronously loads commit details when commit details are expanded.

Screenshots or screen recordings

Before	After
(loads commit details as part of the haml template)	(loads commit details asynchronously when details are expanded)

How to set up and validate locally

1. Enable the load_commit_details_async feature flag
2. Navigate to Repository > Commits
3. Click on the expand button on. commit containing commit details
4. The commit details should load async

Related to #515892 (closed)