[landing] add feedback text to subscription form component

Summary:
As part of the subscription form redesign we need to show some feedback text to the user depending on the result of the email entered in the subscription form. For reference here are the designs:

Depends on D7917

Test Plan:
Please see the screenshots below to see what the feedback text looks like:

Desktop:

Tablet:

Mobile:

Reviewers: atul, kamil

Reviewed By: atul

Subscribers: ted, ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D8039