Backport plan limits

What does this MR do?

This moves all Plan, PlanLimits, Limitable into CE.

This also makes the Plan.default or Plan.free to always return a valid object.

This adds Limitable for ProjectHook and PipelineSchedules on CE.

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