Ship our own webfonts

[Minoru]

#41 highlighted a problem with system fonts: we can't really know how the result will look on the user's computer. This is pretty annoying, since it might degrade readability and we will be none the wiser until someone reports it to us (as happened with #41).

To resolve this, we need to pick a pair of fonts that we'll use, and just ship them to users as webfonts. That puts us in control of how our pages look, and leapfrogs the problems of #41 kind.