[landing] Fix "fluid" font-size for footer:navigation a

Summary: Interpolate from 16px to 24px for width of 320px to 1850px.

Test Plan:
Continues to look as expected:
(towards end)

Reviewers: def-au1t, palys-swm, benschac

Reviewed By: palys-swm, benschac

Subscribers: ashoat, Adrian, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3436