Refresh: update fonts on some Firefox pages

[craigcook]

Description

Thus far we've been intending to switch the fonts globally to Mozilla Headline and Mozilla Text. However, after some discussion with folks on the Brand and Studio teams we've decided to alter that plan slightly:

• "In-product" Firefox pages such as the what's new page and Moment/Welcome pages will retain the Firefox brand fonts (Metropolis and Inter), for better consistency with the rest of Firefox.
• Campaign-specific landing pages such as Challenge the Default, Nothing Personal, and Firefox for Families will retain their brand fonts for consistency with their related campaign materials.
• Everything else on www.mozilla.org (including Firefox marketing pages like /new and /firefox/features) will get the new Mozilla brand fonts (Mozilla Headline and Mozilla Text).