Add support for `/child_epic` quick action when creating a new epic
This is related to:
As part of #288811 (closed) fix we disabled epic-related quick actions when creating new epic (as these quick actions didn't work properly and as a side-effect caused internal_id sync issue). It would be nice to add "proper" support for /child_epic quick action for new epics because this seems to be handy feature.
This is related to #292506 (closed) which tracks addition of /parent_epic quick action, but solutions in both cases might be differet.
We should also make sure that a system note about newly added epic is created on parent epic.