In Review Board 5, we introduced a new SVG-based spinner, which lacked

the wobble of the Font Awesome glyph-based spinner. All of Djblets and

parts of Review Board were updated to use it, but we still had plenty of

code using the old spinner.
This change converts the rest of the codebase to use the new one. It's

basically a drop-in replacement, though it did require some small

adjustments in some parts of the UI to better fit the size or visuals.

In particular, file attachment thumbnails now use margins to position

it, and, given the spinner size, reduce the opacity to make it less

strong.

Tested each spinner in the UI, before and after the change, to make

sure that sizes were still correct.