Option to prepopulate reference fields from current entity

Problem/Motivation

If the view has a contextual filter (or maybe even an exposed or other filter?) allow this value to be passed as a URL parameter, for integration with something like the prepopulate module.

Proposed resolution

Allow a site builder to specify a value to use (use Twig syntax?) and the variable to which it should be passed.

Remaining tasks

User interface changes

API changes

Data model changes