Switch spinners to use Font Awesome's fa-spinner.

Review Request #7173 — Created April 6, 2015 and submitted — Latest diff uploaded

Review Board

Our old spinner.gif is one of the ugliest remaining parts of Review Board. It's
not retina-aware, and it feels pretty out of place in the modern world.

This change swaps out most of the uses of spinner.gif to use Font Awesome's
fa-spinner icon. This uses CSS transforms to rotate the image, and scales
cleanly to any size and resolution.

There are a couple more uses of the old spinner (in rich text rendering and the
new review request/upload diff UIs) which I have yet to clean up. I'll get to
those in a later change.

Looked at each piece of UI (often by making manual changes to the code to
ensure that the spinner stayed on screen long enough to really verify it).