Add additional subepics feature checks

What does this MR do?

It adds subepics feature checks to:

• epic model methods
• graphQL endpoints, presenters and other classes used by graphQL

Following iteration: Move epic to premium section and integrate with frontend

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.

Related to: #37081 (closed)