Bug 1363454 - Create a pref to list icon font families that should be used...

Bug 1363454 - Create a pref to list icon font families that should be used even when use_document_fonts=0, overriding the browser's font prefs. r=emilio,dholbert

Some widely-used icon fonts use ligature rules to replace icon names such as "volume_up"
or "down_arrow" with icon glyphs. If the site is designed to use such a font, but the user
disables document fonts and we use our default Latin font instead, the underlying text will
be rendered instead of the intended icon.

To enable such fonts to continue to work, we provide a list of known ligature-icon fonts
and allow them to be used even when the document-fonts setting is disabled.

Differential Revision: https://phabricator.services.mozilla.com/D167923