[landing] Hello World for landing page

Summary: Hot refresh, Webpack working

Test Plan: Load the loading page at http://localhost/comm/misc/landing.html and confirm hot refresh works if you edit landing/landing.react.js

Reviewers: atul, palys-swm

Reviewed By: atul

Subscribers: KatPo, zrebcu411, Adrian, subnub

Differential Revision: https://phabricator.ashoat.com/D965