#5906737 Use account-specific titles for mailbox list

Changes proposed in this Pull Request

This PR stems from the discussion in #52750, where we were showing “Mailboxes” for all account types in the email plans page.

• The PR adds logic to display “Email forwards” when we’re showing a list of email forwards
• The PR also adds placeholder content while we’re waiting for the data that tells us which account type we have

Testing instructions

• Check out this branch and build it locally, or access the live branch.
• Navigate to /email, and pick a site that has domains with email services, including at least one with email forwarding configured.
• Click on the row for a domain with email forwarding.
• Verify that we show placeholder content in the section header for the list of mailboxes/forwards.
• Verify that we then show “Email forwards” for the section header.
• Click on “Back” to get back to your domain listing.
• Click on one or more domains with paid email services (e.g. G Suite, Google Workspace, or Professional Email)
• For each domain, verify that we show the placeholder for the mailbox list section header, and then show “Mailboxes” once the mailbox list is loaded.

Screenshots

Updated email forwarding header text

Placeholder content in section header (the content in the mailbox list was already in place)