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