R1:6c88c76cacfe

Generate an icon sprite map with new sprites and 2x retina graphics

Summary:
- The filesystem is now the authority for which sprites are available. If you add new icons, the generation process will pick them up.
- I broke out icon generation and added retina support. App icon generation still uses the old method.
- Update ActionList and RemarkupControl to use the new sheet.
- Use white icons on hover.
- Also fixed a couple of minor issues with some stuff in Firefox/Chrome.

Test Plan:
{F25750}…