Dev/build tools: Use grunt-autoprefixer for CSS vendor prefixes.

We'll be using it for two distinct tasks:
 * Core CSS files will keep prefixes. `grunt autoprefixer:core` will update files directly in src/ as a pre-commit step, rather than doing it on build.
 * Color CSS files will receive prefixes when they are built.

This commit:
 * Adds prefixes we were missing to core CSS.
 * Removes prefixes that we no longer need from core CSS.
 * Removes all prefixes from colors CSS.

props ocean90.
fixes #27078.


git-svn-id: https://develop.svn.wordpress.org/trunk@27174 602fd350-edb4-49c9-b593-d223f7449a82