[landing] fix: make footer sticky

Summary:
Make landing page footer stick to the bottom of the page when content doesn't take up all of the screen space

Took this approach: https://css-tricks.com/couple-takes-sticky-footer/#there-is-flexbox

Before:

After:

Test Plan: load landing page, make sure it stays on the bottom

Reviewers: atul, palys-swm, ashoat

Reviewed By: atul, palys-swm, ashoat

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

Differential Revision: https://phabricator.ashoat.com/D2376