Mailbox aliases

Sending a message to `user+alias` should be the same as sending it to `user`, à la Gmail. The alias should be visible from UI. Bonus point if `user+a` and `user+b` are shown as different contacts. This would be useful for message labeling and integrations with external services, e.g. a mail bridge may use `bridgename+contact1`/`bridgename+contact2` instead of registering several ZeroIDs. I'm not sure what the alias format should be, but I think we can just allow any characters (including `@`, this may be useful for bridges too).