WIP: Dry up project feature access logic

What does this MR do?

Dries up some project access logic.

ProjectFeature.feature_available? logic is partially duplicated in ProjectPolicy (or the other way around?), I assume for performance. This makes me slightly uncomfortable. If we dry it up, then they will be perfectly consistent.

• Maybe this is a non-starter, for performance reasons?

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• Documentation created/updated or follow-up review issue created
• Code review guidelines
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• Style guides
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• Separation of EE specific content

Performance 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

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
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