Change title of Humbug Integration to Zulip to reflect company name change. Also update configuration spec.

A few months ago Humbug underwent a name change to Zulip. This change will hopefully bring us up to speed on that front, without requiring that our existing customers reconfigure their hooks.

Also in this pull request we include more configuration fields, to allow much of the flexibility that we've had to special case into our backend code until now.