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:
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Reviewers: chad

Reviewed By: chad

CC: aran

Maniphest Tasks: T2013

Differential Revision: https://secure.phabricator.com/D4027