Fix working with file attachment drafts.
File attachment drafts were a bit broken. The API would return the

non-draft version, meaning it would contain the old caption and not the

new caption. We didn't notice this before the JavaScript rewrite,

because we just showed the local caption we saved, but this is more

apparent now.
The server-side resource now specifically serializes the resulting file

attachment using itself instead of the default serializer.
The JavaScript side now has a DraftFileAttachment object that's used for

all file attachments on the review request page. This is just a thin

wrapper around FileAttachment that makes use of the draft resource.

Set the caption of a file attachment and saw it reflected in the UI.

Verified the correct contents of the request and response payloads.
Reloaded the page and saw that the caption was persistent.
JavaScript unit tests pass. Note that I didn't add any new tests to

test caption saving because the bug was really about the interaction

with resources on the server. Best we can do is simulate what we think

we're going to get.