Use GraphQL for Epics/Issue add action within Epic

Summary

We're relying on Rails API for adding/creating items to Epics Tree (introduced in !10999 (merged)) which doesn't return global ID string for the added item in API response. And since the drag and drop reordering within Epic tree is done via GraphQL mutation, it only works if item has global ID string present.

As a temporary workaround, we're composing the global string on frontend from numeric IDs, which is essentially a hack. We should instead use GraphQL mutation to create/add items to tree as well which guarantees that API response will include global ID string.