Rewrite the review dialog.

The review dialog has had a full rewrite. It's no longer just a
container for a server-side rendered form, which was done in a very
hacky way and could be slow to load for large changes. Now it displays
instantly and fetches the comments itself, rendering them one at a time.

This looks the same as the old form, but with one improvement:
File attachment thumbnails are now rendered as well. They appear just
like screenshot thumbnails do. Previously, we'd only render the
caption or filename.

Unit tests were added for this.

Unit tests pass.

Tested all three types of comments.

Tested modifying every type of field on the reviews and comments and making sure they persisted on save.

Tested publishing.

Tested discarding the draft.

Tested creating a brand new review from scratch with the Review button, and publishing that.