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
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