Resolve "Issue reference changes when adding to epic"

What does this MR do?

Issue #36716 (closed)

• Epic are at a group level, thus it makes sense to always show the full path of an issue (including the project).
• The mismatch between what was shown when adding the issue and after a reload is due to the different mechanisms we use for linking an issue to the epic and loading the initial epic information.
  • To add an issue to an epic, we are doing a POST /groups/:group_id/-/epics/:epic_id/issues. This returns with the LinkedEpicIssueEntity serializer which uses to_reference(full: true).
  • To show an epic after reload, we rely on the GraphQL query. Since our GraphQL already supports the argument full: true, all I had to do is add it to the query.
• I didn't add any specs / change mock data because the change looks trivial and the mock data already included the full path (for some reason).

Screenshots

Before	After

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

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