Allow custom squash commit messages

What does this MR do?

This is the back-end work to allow a user to squash merge requests with their own custom squash message, or choose the message from a list commit messages. It contains a change that makes the default squash commit message to be selected from the first multi-line commit message in the merge request, defaulting to existing behaviour (the title of the MR) if none are found.

The API-specific portion of the issue is in a separate MR https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24784.

What are the relevant issue numbers?

• https://gitlab.com/gitlab-org/gitlab-ce/issues/47149 (front-end issue detailing the feature)
• https://gitlab.com/gitlab-org/gitlab-ce/issues/56014 (back-end, i.e., this MR's issue)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• 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