Add _links to epics API

What does this MR do?

Providing links is common in REST APIs so that you can follow links from resource to resource without needing to manually construct URLS. The self link is particularly useful because it allows a collection to be iterated over, and actions to be performed using the self URL to modify or update the issue.

Links are already provided by other API resources like Issues, Merge Requests and more, so this improves consistency.

Adds _links to the Epics API.

"_links": {
  "self": "http://127.0.0.1:3000/api/v4/groups/22/epics/5",
  "epic_issues": "http://127.0.0.1:3000/api/v4/groups/22/epics/5/issues",
  "group": "http://127.0.0.1:3000/api/v4/groups/22"
}

Closes #230586 (closed)

Screenshots

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

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
• Security reports checked/validated by a reviewer from the AppSec team