Drag and drop output ref

Spec: https://docs.google.com/a/dimagi.com/document/d/1TjJ3vUjH3Xc_dkYd8pKoErVFjcXkbsYlfClSlcJ9Ic0/edit#

@millerdev 
Also, the [coding style guide](https://github.com/airbnb/javascript#variables) for Vellum typically puts all variables in a single `var`.