Now that we have a new, improved field for specifying close descriptions

on a review request, it's time to make use of it in the UI, along with

cleaning up the code surrounding it.
The code dealing with "changeDescription" has been split up a bit to

work with either "changeDescription" or "closeDescription," and to look

in the ReviewRequest or DraftReviewRequest, depending on the field.
This also helps set up for bits of the new server-side Markdown change.

Edited draft change descriptions and close descriptions.
Unit tests all pass.