Limit font queries per URL bar domain

In legacy/trac#13313 (moved), we introduced a patch to restrict the fonts allowed to be loaded in Tor Browser. But different versions of the same font could still be used to distinguish users. We could potentially limit the damage by introducing a second patch that restricts the number of allowed font requests per URL bar domain.

Previously we had a patch for legacy/trac#2872 (closed) that worked something like this, although it wasn't tied to URL bar domain.