It's now necessary to add a `converse-root` element in the DOM where you
want Converse to render (previously it was any element with the id
`#conversejs`).
Also, turned `converse-chats` element into a Lit element and re-render
`converse-root` and `converse-chats` when the `view-mode` or `singleton`
settings change. This is a step towards being able to change the view
mode on the fly and have the entire chat re-render appropriately.
Fixes#2647
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
