In the case of the close description field, the way that the review request

editor would call reviewRequest.close instead of setting a field on the model

itself was pretty hacky. The implementation of setDraftField would look at

whether it was setting the closeDescription and then special-case its

behavior.
Now that we have JavaScript-side implementations for the built-in fields, we

don't need this anymore. I've moved the special casing into the

CloseDescriptionFieldView.
This refactor will also make things easier down the road for new field types.

Tested setting the close description with both regular and Markdown text.
Ran js-tests.