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.