#6669008 Add/jetpack sale coupon

Changes proposed in this Pull Request

Context: pbNhbs-16p-p2

• Adds the data layer and redux store for the Jetpack Coupon Sale
• Adds a Sale Banner on cloud.jetpack.com/pricing
• Removes the Introductory pricing banner if there’s a sale running
• Changes the discount percentage based on the discount of the sale (ideally this should use the sale_cost property of a product, but it’s not on the scope for this change)

Note: I’m a bit uncertain about removing the introductory pricing banner, because it will be shown until we fetch the data from the backend, and if there’s a sale running it would cause the frontend to change abruptly when hiding the banner

Screenshot for reference:

Testing instructions

Unit tests

• For unit tests: yarn test-client client/state/marketing/test

Functional test

• Follow the steps 3 and 4 in D66708-code, and sandbox the API
• Checkout this PR locally
• Run both yarn start and yarn start-jetpack-cloud-p. Another option is to only start Calypso Blue through yarn start and use the live link for Jetpack Cloud appended with ?calypso_env=development
• Go to http://jetpack.cloud.localhost:3001/pricing?calypso_env=development
• Make sure a sale banner appear at the top and the countdown timer works
• Confirm the introductory pricing is hidden if the sale is running
• Make sure the prices in the pricing page has a discount equivalent to what is returned by the API call, and this price is also applied at checkout