Troubleshooting wizard for own mailbox being unreachable

Design and implement a troubleshooting wizard to guide the user through the steps of diagnosing why their own mailbox is unreachable and fixing the problem.

The wizard will be reachable from the mailbox status page (#2172 (closed)) when automatic reachability checks repeatedly fail, or when the user triggers a manual reachability check which fails.

The UI for warning the user that their mailbox is unreachable (#2175 (closed)) will guide the user to the status page where they'll be able to use the wizard.