[landing] [fix] footer layout on mobile

Summary:
before:

after:

side stepping normal diff review because we don't want a broken signup experience this week.

Test Plan: shrink browser, it should not look broken

Reviewers: atul, ashoat

Reviewed By: ashoat

Subscribers: ashoat, palys-swm, Adrian, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3232