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

package.json

@@ -21,6 +21,7 @@
     "grunt-contrib-jshint": "~0.8.0",
     "grunt-cssjanus": "~0.2.2",
     "grunt-sass": "~0.10.0",
+    "grunt-autoprefixer": "~0.6.5",
     "grunt-jsvalidate": "~0.2.2",
     "grunt-contrib-imagemin" : "~0.4.1",
     "matchdep": "~0.3.0"