[server] Use React SSR for landing page

Summary:
We could actually directly SSR landing/landing.react right now instead of going through Webpack with landing/dist/landing.build.cjs, which works through the Webpack config introduced in D965.

This issue is that CSS modules wouldn't work. They're not in use right now, but they will be soon.

Test Plan: Load the website at http://localhost/commlanding/ on both dev and prod

Reviewers: atul, palys-swm

Reviewed By: atul

Subscribers: KatPo, zrebcu411, Adrian, subnub

Differential Revision: https://phabricator.ashoat.com/D971