Drag and drop output ref

Spec: https://docs.google.com/a/dimagi.com/document/d/1TjJ3vUjH3Xc_dkYd8pKoErVFjcXkbsYlfClSlcJ9Ic0/edit#

@millerdev
`startsWith` is only available in the 'strings' plugin for underscore, not in the core underscore lib.