Resolve "Related Issues BE followup - Add public API"

Merged Oswaldo Ferreir requested to merge 2633-related-issues-public-api into master

What does this MR do?

Adds endpoints for listing, creating and deleting issue relations.

Does this MR meet the acceptance criteria?

What are the relevant issue numbers?

Closes #2633 (closed)

Edited by Oswaldo Ferreir