We had a few places where we had defined LESS functions to add vendor prefixes

to CSS. This wasn't entirely consistent (for example, we had a mix of the

standard opacity and our own .opacity()), and most of it was out of date.
This change pulls in less-plugin-autoprefix, which uses the very nice

autoprefixer project to automatically rewrite any rules which require prefixes

based on which browsers we want to support. For now, I've made that list of

supported browsers be anything which has more than 2% market share, plus IE 9

and 10 (which are each about 0.8% right now). I wouldn't mind dropping support

for those older IEs, but that's a separate decision.
The one thing which autoprefixer doesn't do is add filter() for linear

gradients in IE9. I've left those in for now, but if we drop IE9 for 2.6+, then

those can go away too.

Reloaded and checked that static media was rebuilt. Looked in the compiled .css

files and saw that things which required browser prefixes (such as transform

being rewritten as both transform and -ms-transform) were handled as

expected.