Add the beginnings of the ReviewRequestEditor and view.

Right now, this just tracks thumbnails and general editing/publishing
state. Most of the real logic still exists in reviews.js.

In time, this will handle all aspects of review request editing.

Unit tests were added for all the current event hookups, rendering,
thumbnail importing, and more.

Tested pages with existing screenshots and file attachments. Made
sure I could edit the captions and delete the screenshots.

Tested uploading new file attachments (through drag-and-drop and through
Add File). Tested that I could edit and delete those.

Tested that edit state seemed to still work on fields.

Tested the draft banner when saving fields and uploading.

Unit tests passed.