Add font-display property to glyphicon's font-face

Created by: wmcmurray

This change will remove this Google Lighthouse's warning for the glyphicon font :

Ensure text remains visible during webfont load

• their page on the subject
• the file where this check is done

Any value (except auto) for the font-display would work I guess, but block seamed the proper choice since we don't want the browser trying to display glyphicons with another system font until the real font gets loaded !

• font-display documentation

Thanks for your time !