Our bundled copy of Open Sans doesn't include italics or bold weights

[callahad]

Our copy of OpenSans at _static/OpenSans.woff2 only has the regular variant. We need to include bold, italics, and probably bold+italic. Otherwise the browser has to guess at how those glyphs should look, and it does not do a good job.

Our @font-face rules are in source/theme/layout.html

[callahad]

Also, everyone supports woff2; we can drop the oldschool woff.