Resolve "Override squash commit message"

CE-counterpart of https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9464

What does this MR do?

Add a possibility to have a squash commit with editable commit message when choosing to merge MR squashing commits.

Squash commits enabled

Collapsed	Expanded	Dropdown

• Although modifying the author of the squash commit is out of scope for this issue, I've included it in the mockup above to get an overall idea of the feature.

• The "Use an existing commit message" dropdown allows the user to pick an existing commit message to replace the text in the textbox.

Squash commits not enabled

Collapsed	Expanded	Hover

• The UI for modifying merge commit has been changed for the sake of consistency.

• The include MR description toggle link has been changed into a checkbox which is a better design pattern.

• The hover color of the gray bar is #CCCCCC.

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)