Prevent merge if denied licenses are found

What does this MR do?

Implementation of #215891 (closed)

1. Calculate if MR has found denied licenses
2. Make it not mergeable
3. Add calculated value to /cached_widget.json response
4. Cover changes with tests and small refactoring of #mergeable? spec

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