Prevent editing commits if signed commits required

What does this MR do?

On &4983 (closed), we want to allow editing commit messages on unprotected branches. Until a better solution, we can't edit commit messages that are signed, so if "Required signed commits" is enabled, this option shouldn't be enabled.

This commits prevents for editing commit setting to be enabled if signed commits option is required.

Editing commit option is under a feature flag disabled by default, as such UI is not visible on GitLab.com.

Related to #292493 (closed)

Screenshots (strongly suggested)

Validation error

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