Fix diff loading when there's a draft.

I was using "review_request_details.get_absolute_url" as the diff base URL in
the view_diff.html template, which worked fine for published reviews but broke
when there was a draft, because ReviewRequestDraft doesn't have a
get_absolute_url implementation. I've changed it to plumb through a base_url
parameter, just like the chunk loading code.

Looked at a draft diff in a local site.