Support GitHub style task breakdowns to create epics

[Nico Rehwaldt]

Is your feature request related to a problem? Please describe

GitHub recently introduced the ability to break down tasks into sub-issues right from a task list:

* [ ] ABC

---

* [ ] ABC #SOME_ISSUE_NR

Detecting such links gives us the potential to automatically establish parent ↪️ child relationships between issues.

Describe the solution you'd like

Rather than requiring a manual linking of sub-tasks via Child of #ISSUE_NR we shall determine that link automatically.

Describe alternatives you've considered

Do nothing and keep the requirement of manual linking.

Additional context

Example issue: camunda/camunda-modeler#2488