[landing] Add SubscriptionForm to HeroContent

Summary:
Add SubscriptionForm to HeroContent to make it more visible on the landing page.

Next will get rid of the Request Access button in the Header

Test Plan:
Looks as expected: https://blob.sh/atul/309e.png

Checked some other breakpoints to make sure things look good/proportional

Reviewers: def-au1t, palys-swm, varun, benschac

Reviewed By: palys-swm

Subscribers: benschac, ashoat, Adrian, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3401