Email newsletter: Design the User signup experience

[clementine-smart]

As a user, I want to be able to opt in to receive email newsletters relevant to a particular garden. This will be done from the "About" page for a garden.

Scope Note that this ticket encompasses the UX / UI design only - to do be done in figma.

In the Figma file, design this interface including the following considerations:

• [x] How might this experience vary if the user is logged in or not logged in?
• [x] What will a user see when their subscription is successful or if it fails?
• [x] What will a user see if they are already subscribed to a garden when they arrive on the page?

STRETCH:

• [ ] How might a user unsubscribe from a garden?

When designs have been approved by the client, move this ticket to done. This issue unblocks #240