Mailbox aliases

[purplesyringa]

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).