[landing] fix css class names for subscription form component

Summary:
Clean up diff to change naming convention of css classes for SubscriptionForm from snake case to camel case. (All other css files in landing uses camel casing)

Depends on D7915

Test Plan: Everything works as expected

Reviewers: atul, kamil

Reviewed By: atul

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D7917