Using an external reference to h.woff rather than an inline font
enables the font to load when served by the Chrome extension
on sites that have a strict Content Security Policy, such as
GitHub.

Regarding the removed comment about URL rewriting, the
new Gulp-based build system currently doesn't do any
URL rewriting but we do know that styles
will be served at /(assets|public)/styles/<bundle name>.css
and fonts at /(assets|public)/fonts/<name>.woff , so
we can just use a URL relative to the CSS file path.

Fixes #2266