refactor(chat): Flatten rules in chat.scss

[gadenbuie]

Small refactor to flatten nested rules in chat.scss, since the custom element rules don't need to be nested.

Flatter rules are easier to modify from userland and I contend they're easier to maintain because we retain nesting where class and element interactions are important.

[cpsievert]

The inspiration for nesting is two-fold:

1. Reduce the chance of clashing with other rules
2. Provide a signal that the nested elements should only exist inside certain parent elements

That said, I don't feel those things necessarily trump simpler selectors in this case, so OK with me