Epic and Task hierarchy

[martinsangolt]

In order to make Github Boards really useful as a Kanban tool — we need better Epic - Task hierarchy linking. It is really difficult to track or understand the progress of larger chunks of task with a completely flat issues structure.

[feerrenrut]

This seems to be described on the public roadmap, but without a specific timeline (only 'future'): github/roadmap#284

See also (currently the most upvoted discussion I could find): #5714