Support creating a child epic from API

What does this MR do?

It changes POST epics API end point which now accepts parent_it parameter.

Couple of points to notice:

• code that checks if an epic is valid has been moved from the EpicLinks::CreateService to the Epic model
• we use validate_parent only on the creation of an epic. When we add another epic to an existing epic we use EpicLinks::CreateService for it and there we first reject all epics that are not valid and can be related to the parent one. We'd do the check twice

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

If cross-browser testing is not required, please remove the relevant item, or mark it as not needed: [-] -->

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

#7331 (closed)