Automatically inline small images using data URIs

Summary: Fixes T3300. Images under 32KB are inlined automatically into CSS using data URIs; larger images remain as normal links. I picked the 32KB threshold arbitrarily, based on it looking roughly like it got reasonable results on core.pkg.css (inlining most of the random stuff, but not inlining all the 2X sprites and such).

Test Plan: Loaded site, browsed around, looked at generated CSS.

Reviewers: btrahan, chad

Reviewed By: chad

CC: aran

Maniphest Tasks: T3300

Differential Revision: https://secure.phabricator.com/D8225