[landing] Bring back "fluid" font-size to <p>

Summary:
Basically moved html.font-size to p.font-size

Before:

depends on D3430

Test Plan: Checked <p> tags and made sure that they look as they did before removing the html font-size clamp

Reviewers: benschac, palys-swm, ashoat

Reviewed By: benschac, ashoat

Subscribers: Adrian, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3432