Importing a plugin's `index.js` file from other plugins means that it's no
longer possible to remove that plugin by removing it from `src/converse.js`.
Instead, all plugins need to be imported in proper order from
`src/headless/headless.js` and `src/converse.js`.
Updates #3026
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
