Add split button to create issue from epics tree

What does this MR do?

• Introduce new epic_new_issue feature flag.
• Add new shared split button component.
• Use split button to toggle create issue form on epics tree.

part of #5419 (closed)

Screenshots

Screen_Recording_2019-10-17_at_20.47.41

Manual testing

The changes are currently behind two feature flags:

bin/rails console

Feature.enable(:epic_trees)
Feature.enable(:epic_new_issue)