[landing] Add thumbnail and description for landing page when shared

Summary:
Add image thumbnail and website description (from Twitter bio) for the landing page so it appears in Twitter Cards/Facebook messages/posts when shared.

Related Linear issues with full context: ENG-1066.

Test Plan:
N/A, as of now. There is no staging environment with a public URL so not sure how to test these changes yet. This is tracked by ENG-889 (FB called this "Sandcastle").

Once there is, though:

• The Twitter Card Validator tests how URLs look on Twitter Cards when Tweeted.
• Facebook has its own Markup Tester to see how URLs look on Facebook posts when shared. I'm not sure if this works for Messenger as well, but that can be tested by sending a message to someone.

Reviewers: atul, ashoat

Reviewed By: atul, ashoat

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

Differential Revision: https://phabricator.ashoat.com/D3886