Add option to hide the default "thumbs up" and "thumbs down" buttons on issues, merge requests, and snippets.

What does this MR do?

Under "Project > Settings > General > Visibility, project features, permissions", adds a "Show default reactions" checkbox. Defaults to enabled, to match existing behavior.

Based loosely on 45c33bcb

Screenshots

New project setting:

When checkbox is enabled:

When checkbox is 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

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Closes #211519 (closed)