Show a spinner before rendering markdown text or diff index.
When displaying a review request, there's a brief moment of time before

Markdown gets rendered or before the diff table is loaded where things

look wrong. For the Markdown fields (namely the Description and Testing

Done fields), the raw Markdown is shown first, and then switches to the

rendered text, which looks bad. The diff index shows a big line before

displaying its contents.
These now are marked as loading at first, and we'll show spinners. Once

the page has successfully loaded, the spinners go away and the rendered

content appears.
We also try to render the text for these fields a bit sooner than we did

before, hopefully reducing the load time.

Tested the description, testing done, and change description fields,

and the diff index, as the owner of the review request and as anonymous.
The page started out with those fields showing spinners, and the approximate

text size (since the raw text was loaded), and then soon after, it was replaced

with the rendered text.
Same with the diff field. It just showed a spinner roughly where the diff file

entries would go, and then the whole thing loaded. It was a bit weird with the

"Diff Revision ..." appearing, but not as bad as it was before.