Conceal system fonts from web sites

[riastradh-brave]

System fonts are a standard fingerprinting vector. Web sites that need specific fonts can bundle them with the site; otherwise they will look different, and likely bad, on different computers anyway. We should always set the system fonts -- as available for use by name in CSS, and anywhere else they might appear -- to some standard baseline set so that users cannot be distinguished by it.

I would like to do this always, but if there is a compelling reason it has to be limited to when fingerprinting protection is enabled, so be it.

[tildelowengrimm]

Even though this one came first, I'm duping this into #816 because there's more discussion there.