[landing] redesign the subscription form component

Summary:
As part of the Hero redesign for the landing page we need to redesign our SubscriptionForm component. The focus of this diff is to change the language and visuals (color and layout by separating the input field from the button) shown on SubscriptionForm. Here is a screenshot of the Figma doc (please note that a subsequent diff will handle displaying the feedback text shown in the Figma)

Depends on D7914

Test Plan:
Please see the screenshots below:

Desktop Normal:

Hover:

Success Case:

Error Case:

Mobile View:

Tablet View:

Reviewers: atul, kamil

Reviewed By: atul

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D7915