#5759492 Add mailbox list placeholder in new email list

Changes proposed in this Pull Request

• The core goal of this PR is to add a loading indicator to the list of mailboxes in the domain-specific email management screen, and to show a basic “No mailboxes” message when no emails are specified.
• While working on the component, I refactored it to be functional, and also included some internal helper components to ensure we use the same structural elements for all paths through the code.

Note: This PR is based on PR #51452, so needs that change to be reviewed and merged first.

Testing instructions

• Either load (http://calypso.localhost:3000) or the live branch, and then navigate to /email/:siteSlug?flags=email/centralized-home for a site with test domains and email subscriptions.
• For some of your domains with email, click on the row and verify that you see a loading row for the mailbox section, and then the list of mailboxes.
• To verify the “No mailboxes” message, you may need to interfere with the REST API or the code to simulate that situation. You may also have one or more WordPress.com Email accounts where you haven’t created any mailboxes yet, which would also test this case.

Screenshot