feat(tree list): nested tree list form
VIP - will be rebased

needs to be fixed implementation (one falling test)
fix(tree): fix tests
Merge branch 'tree'

Conflicts:
	Gemfile
	spec/dummy_app/app/assets/stylesheets/application.css
	spec/dummy_app/app/models/project.rb
	spec/dummy_app/db/schema.rb
	spec/spec_helper.rb