Epics API does not have _links

Problem to solve

The Epics API is inconsistent with the Issues and Merge Request API and does not have links. The _links attribute is very useful when using a restful API. Particularly the self link can be quite useful when taking a collection, and then performing the same POST or PUT action on all the items in the collection.

Further details

It would be nice if all issuable API entities were consistent, including _links so that it's easier to extend scripts that work with issues and merge requests, to also work with epics.

Proposal

Implement basic _links support in the Epics API for consistency.