When there is a circular dependency between disco entities (via their
advertised `disco#items`), Converse went into an infinite loop because
even though there was a check whether an entity already existed, it
failed to add newly created entities to the global
`_converse.disco_entities` collection.
The eventual goal is to avoid UI-related stanza processing if the relevant chats
aren't in the DOM.
With the current architecture, chatboxes are created (and the stanzas
related to them processed) even if `#conversejs` isn't in the DOM.
* Initial work on making controlbox an element
* Create a shared base class
* Ceate ChatBoxViews proxy
* Update sass now that certain classes are moved to converse-chats element
