WIP: Override squash commit message

What does this MR do?

Add a possibility to have a squash commit with editable commit message when choosing to merge MR squashing commits. Add some commit details compared to previous UI (committer data):

Squash	No squash

What are the relevant issue numbers?

#47149 (closed)

#56014 (closed)

#52278 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer

Closes #47149 (closed)