Add support for relating epics

Related to #351597 (closed).

Add required service and controller to be able to relate epics using an internal endpoint.

The new service should inherit from IssuableLinks::CreateService and match the functionality of IssueLinks::CreateService.

Similarly to IssueLinksController, the new controller should include the functionality of IssuableLinks.

We should create system notes when relating epics. See SystemNoteService#relate_issue.

This feature would be developed behind a feature flag and needs to perform license checks (implemented in #352439 (closed)).

Existing functionality for relating issues:

POST /*namespace_id/:project_id/-/issues/:issue_id/links

Example Request Payload

{"issuable_references":["#1008"],"link_type":"blocks"}

Example Response Body

[{"id":1549,"confidential":false,"title":"New Issue Title 967","assignees":[],"state":"opened","milestone":null,"weight":null,"reference":"#1003","path":"/gitlab-org/gitlab-test/-/issues/1003","due_date":null,"created_at":"2021-03-11T10:49:42.124Z","closed_at":null,"relation_path":"/gitlab-org/gitlab-test/-/issues/1037/links/9","link_type":"relates_to"}]

Given that a similar endpoint alreayd exists for epics POST /groups/*group_id/-/epics/:epic_id/links(.:format), we have to consider a different name.

For example POST /groups/*group_id/-/epics/:epic_id/related_epic_links(.:format)

Additional notes

• Also add (if needed) relation_path to ee/app/serializers/epics/related_epic_entity.rb (this couldn't be added as part of #353472 (closed) because this route was not defined yet, but it will be probably needed by FE for sending create/destroy requests

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