Update fonts and switch to Medium font on the Mac. 

Also, use grayscale antialiasing instead of subpixel antialiasing on the Mac.

This is to fix animation flicker as in #1993. To test it, you can do a Find in Files--without the new fonts/antialiasing change, you should see some parts of the UI (typically the line numbers, but sometimes the whole code editor) flicker lighter on the Mac; with the new fonts, there should be no flicker.

Note that I actually created a new `font-family` (`SourceCodePro-Medium`) instead of just defining a new weight of `SourceCodePro`--I wasn't able to get the latter to work properly.