[landing] Introduce QR landing page

Summary: Page that ETHDenver QR code will redirect to with links to our Twitter page, GitHub repo, etc.

Test Plan:
Looks as expected: https://blob.sh/atul/10a5.png

Looks a little weird on desktop: https://blob.sh/atul/7b93.png, but I think that's fine since I'm guessing 99+% of people will be hitting this page from a mobile phone.

Reviewers: varun, benschac, ashoat

Reviewed By: benschac, ashoat

Subscribers: palys-swm, Adrian, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3235