Specify list container styles with a class

Add a `gi-custom-container` class to the wrapper for applying the default styles to the user list.

These styles can be overridden like so:

```css
.gi-webrtc.gi-override.gi-custom-container {
  position: relative;
}

gi-webrtc.gi-override .gi-collapse-wrapper {
  display: block;
}
```