Introduce a text field that enables people to offer context about why they declined to add a reference when Edit Check invited them to do so.

Story

As someone who declined to add a reference when Edit Check invited them to do so and who did so for reason(s) not represented within the default multiple choice options, I'd like to be able to share why I made the choices that I did, so that the person/people who will eventually see the edit I'm making will be less likely to undo the change I've made.

This ticket was inspired by the ideas @Sdkb shared on mediawiki.org.