[fix] [landing] address header media query sizes

Summary:
Add a tablet sized break point for the header
Mobile and Tablet

Desktop

Test Plan: re-size the browser, view on a mobile device

Reviewers: atul, ashoat

Reviewed By: atul, ashoat

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

Differential Revision: https://phabricator.ashoat.com/D2656