Extract featurable concern

What does this MR do?

We extract some logic out of ProjectFeature, to be shared with the planned GroupFeature model.

Part of #208412 (closed)

Based on gitlab-foss!15060 (closed) by @randaalex

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.