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
Fixes#1083
Directives are rendered as templates and their bodies are MessageText instances.
We thereby achieve the necessary nesting of directives (and other rich
elements inside directives) by letting each directive
body render itself similarly to how the whole message body is rendered.
