Changing the caption on a file attachment/screenshot that was present on

page load would not result in a draft banner appearing until the page

was next reloaded. This happened because we didn't register any of the

file attachments or screenshots that were provided as part of the page

load, preventing their events from being connected with the

ReviewRequestEditor's events.
This change registers existing attachments/screenshots and adds unit

tests to ensure this works and does not regress.

Edited the caption on a file attachment on a review request without a

draft. Saw that the draft banner appeared, whereas prior to this it

would not. Repeated this test for screenshots.
Unit tests pass.