Unify the theme-browsing and theme-customization experiences by introducing a comprehensive theme browser and installer directly accessible in the customizer. Replaces the customizer theme switcher with a full-screen panel for discovering/browsing and installing themes available on WordPress.org. Themes can now be installed and previewed directly in the customizer without entering the wp-admin context. Also includes an extensible framework for browsing and installing themes from other sources.
Also includes CSS auto-prefixing added via `grunt precommit:css`.
For details, see: https://make.wordpress.org/core/2016/10/03/feature-proposal-a-new-experience-for-discovering-installing-and-previewing-themes-in-the-customizer/
Previously [38813] but reverted in [39140].
Fixes#37661, #34843, #38666.
Props celloexpressions, folletto, westonruter, karmatosed, melchoyce, afercia.
git-svn-id: https://develop.svn.wordpress.org/trunk@41648 602fd350-edb4-49c9-b593-d223f7449a82
c35fe07703