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
before `converse.initialize` has been called
In the process, remove other "core" promises. There is no core or
community plugins code awaiting on them and since it's possible to
call `converse.initialize` twice, I'm not sure it makes sense to even have them.
Also removed unused error class IllegalMessage.
This element can be used to declaratively insert Converse into the DOM.
You can put `<converse-root>` into the DOM (e.g. by rendering it inside
a React component) before or afer Converse has loaded and/or has been
initialized.