Refactor MailboxProperties to hold raw onion address
This branch refactors the MailboxProperties class to hold the raw onion address of the mailbox, rather than the base URL. This avoids needing to have multiple places in the codebase that convert between onion addresses and base URLs.
The remaining conversions are done by a UrlConverter class that can be replaced in tests, allowing us to test against real mailboxes running on localhost.