Add support for user-defined unified mailboxes

We still default to Inbox, Sent and Drafts but users now can defined
their own unified mailboxes or they can exclude (or include) additional
collections into the default unified mailboxes.

I think reacting to changes does not work properly, the notification
subscription must be checked.

The BoxManager needs likely another look, probably could be smarter
and maybe built on top of custom Monitor (for Collections) to better
react to changes.