[landing] make footer flush with the bottom of the screen

Summary: Going through the landing page one more time and found this bug on large screens where the footer was not flushed with the bottom of the screen. This diff fixes this issue

Test Plan:
Please see screenshots below

Before:

After:

Reviewers: ashoat, inka

Reviewed By: ashoat

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12633