#5471210 Add Titan terms of service to checkout

Changes proposed in this Pull Request

• This PR adds the collective Titan terms of service that users need to agree to whenever a Titan product is present in the cart/checkout.

Testing instructions

• Ensure that you’re running your back end with Store Sandbox enabled.
• Ensure that you’re running Calypso with the titan/phase-2 feature flag enabled.
• Add some number of Email mailboxes to your cart (try with a new subscription and with additional mailboxes; also try with other products).
• Verify that the checkout page shows the following text when one or more Titan mailboxes are being purchased:

You understand that your Email service will be powered by Titan and is subject to Titan’s Customer Terms of Service, Acceptable Use Policy, and Privacy Policy.

• Ensure that the links work and open the correct documents in a new window.

I am still confirming whether the documents are available in any other languages. (Confirmed: they are only available in English.)