Had to make various other changes due to incompatibilities.
- Use the new `lit` package instead of `lit-html` or `lit-element`
- Drop `haunted` since it breaks the rules by specifying `type: module`
but then doesn't import with file extensions
- Use Sass Dart instead of node-sass (fixes#2445)
- Upgrade Karma
- Add test for incoming RAI message
- Only enable RAI if the user is affilated in MUC being left
- Handle error presence indicating a resouce-constraint
- Don't unregister stanza handlers in `leave`, since we still want to
listen to RAI-related stanzas. Instead unregister upon the `destroy`
event.
