#5447065 Show more details in the Email quantity selection UI

Changes proposed in this Pull Request

This change adds more details to the quantity selection UI to give users clearer context. Those changes are:

• Always show which domain we’re working with, both in the header bar and in the main page body.
• If you have already bought mailboxes for the domain, and you have all those mailboxes set up, we show a message that simply describes the number of mailboxes you already have: You currently have 2 mailboxes for this domain

• If you have already bought mailboxes for the domain, but you haven’t configured all the mailboxes yet, we show two messages:
  • You have already bought 3 mailboxes for this domain
  • You still have 1 unused mailbox

Testing instructions

Ensure that you have Store Sandbox enabled and that the titan/phase-2 feature flag is enabled.

Then check the following cases to make sure things look good:

• Start the process of adding Email to a domain that doesn’t have it yet. The only changes should be showing the domain in the header bar and the card body.
• Try to add mailboxes to a domain that has purchased Email, but not all mailboxes are configured. Ensure that you see both the message about the total number of mailboxes and the number of unused mailboxes.
• Try to add mailboxes to a domain that has purchased Email, and all of the mailboxes are configured. Ensure that you only see the message about the number of mailboxes.