Add form to create issues to epics tree feature
What does this MR do?
Adds a new form to the epics tree feature which allows to create new issues in a project of the group or any subgroup.
part of #5419 (closed)
Screenshots
Manual Testing
The changes are currently behind two feature flags:
bin/rails console
Feature.enable(:epic_trees)
Feature.enable(:epic_new_issue)
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