Inform user that their camera is starting in Lobby (#2869)

* Inform user that their camera is starting

Instead of just showing a grey box.

* Review feedback

* Show spinner from design suggestion

* useMemo

* Lint

* Lint

* Feedback from review

* Use colour that actually exists

* Refactor into Avatar superclass

* .

* Remove size limit behaviour

* Add VideoPreview tests

src/Avatar.tsx

@@ -33,7 +33,7 @@ export const sizes = new Map([
   [Size.XL, 90],
 ]);
 
-interface Props {
+export interface Props {
   id: string;
   name: string;
   className?: string;