Add a project setting to allow edit commit messages

What does this MR do?

Add a project setting to allow edit commit messages

A new setting was added under Project > Settings > General > Visibility, project features, permissions to allow commit authors to edit commit messages on unprotected branches. This setting is stored in a new column in project_settings table

Changes are currently under a feature flag.

Related to #287798 (closed)

Screenshots (strongly suggested)

With feature flag enabled	With feature flag disabled

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done