WIP: Resolve "Include description in commit message" not working immediately after creating MR"

What does this MR do?

Delays the initialization of MergeRequest javascript until after DOM ready.

Why was this MR needed?

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. The bug was encountered as part of the approval UI upgrades in EE (https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/954#note_21565851) and also reported in #26780 (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)

cc: @smcgivern @adamniedzielski