[landing] Flow updates

Summary:
These updates are necessary to make landing page work with flow-bin@0.137.0, which is the version associated with react-native@0.64.0.

Wanted to submit them now to avoid having to deal with any rebasing later.

Test Plan: Flow

Reviewers: atul

Reviewed By: atul

Subscribers: KatPo, zrebcu411, palys-swm, Adrian, subnub

Differential Revision: https://phabricator.ashoat.com/D973