Use GraphQL API for issue links
In !20617 (merged) and !21375 (merged) we added support for setting a type of an issue link (when adding related issues). This is sufficient for #2035 (closed) because frontend currently uses internal API.
Currently we don't have API for adding related issues in GraphQL, it would be great to add support for issue links to our GraphQL API too
Let's also update the frontend to use GraphQL for linked issues.
/cc @johnhope - this probably doesn't have the highest priority (as there is currently no feature which would depend on it)
Edited by Donald Cook