This change adds initial support for change descriptions for review request drafts. When the user goes to make a change, they can optionally add text describing what changed.

Changes made (with or without accompanying text) will be displayed on the review request page, allowing users to see that, for example, a new diff has been uploaded.

Further improvements down the road include showing thumbnails of changed screenshots and offering interdiff links for the new diff.

Made all kinds of changes to review requests and saw the expected results on each. The unit tests also pass.

Selenium tests down the road will help verify that this works as expected.