#5610536 Update email comparison page layout

Changes proposed in this Pull Request

• This PR replaces #51062 with an implementation that uses a new EmailProvidersStackedComparison component to render the new email comparison page layout and embed the relevant creation forms for Email and Google Workspace.
• The design is in pcDL0W-79-p2, and this hews fairly closely to those designs.
• Note that this PR is based on #51298, which extracts the feature lists for the various email providers.
• The feature is protected by the titan/provision-mailboxes feature flag.

Screenshots

Email expanded (the default)

Google Workspace/G Suite expanded

Email Forwarding expanded

Testing instructions

Navigate to the live branch, and then add ?flags=titan/provision-mailboxes to the URL.

• Try to add each of Email, Google Workspace, and Email Forwarding to your account.
• For each provider, check that the error handling is working, and that we track clicks on the expand buttons as well as the “Add ” buttons.
• You should verify that the shopping cart entry created for Titan and Google Workspace includes the correct user/mailbox data. (We are addressing some back end bugs for Titan, so full testing here may trip over those issues.)

Related to #51062
Related to #51298