Backport of EE - delays the initialization of MergeRequest javascrip…

What does this MR do?

Backport of EE fix in !954 (merged) - delays the initialization of MergeRequest javascript until after DOM ready.

Fixes a bug where the click handler for the "Include description in commit message" button on Merge Requests was not being properly registered in some cases.

This bug is fixed in EE but not in CE.

Why was this MR needed?

Fix is already in master, but we can't merge the commit where it was made into 8.16, because it's the one that removes turbolinks.

See this MR and the discussion in the comments for further details: !8628 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #26780 (closed)