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/tile/TileAvatar.test.tsx

@@ -0,0 +1,27 @@
+/*
+Copyright 2024 New Vector Ltd.
+
+SPDX-License-Identifier: AGPL-3.0-only
+Please see LICENSE in the repository root for full details.
+*/
+
+import { expect, describe, it } from "vitest";
+import { render } from "@testing-library/react";
+
+import { TileAvatar } from "./TileAvatar";
+
+describe("TileAvatar", () => {
+  it("should show loading spinner when loading", () => {
+    const { container } = render(
+      <TileAvatar id="@a:example.org" name="Alice" size={96} loading={true} />,
+    );
+    expect(container.querySelector(".loading")).toBeInTheDocument();
+  });
+
+  it("should not show loading spinner when not loading", () => {
+    const { container } = render(
+      <TileAvatar id="@a:example.org" name="Alice" size={96} loading={false} />,
+    );
+    expect(container.querySelector(".loading")).not.toBeInTheDocument();
+  });
+});