Start refactoring some business logic into view models

As Element Call grows in complexity, it has become a pain point that our business logic remains so tightly coupled to the UI code. In particular, this has made testing difficult, and the complex semantics of React hooks are not a great match for arbitrary business logic. Here, I show the beginnings of what it would look like for us to adopt the MVVM pattern. I've created a CallViewModel and TileViewModel that expose their state to the UI as rxjs Observables, as well as a couple of helper functions for consuming view models in React code.

This should contain no user-visible changes, but we need to watch out for regressions particularly around focus switching and promotion of speakers, because this was the logic I chose to refactor first.

src/state/subscribe.tsx

@@ -0,0 +1,38 @@
+/*
+Copyright 2023 New Vector Ltd
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+import { FC } from "react";
+// eslint-disable-next-line no-restricted-imports
+import { Subscribe, RemoveSubscribe } from "@react-rxjs/core";
+
+/**
+ * Wraps a React component that consumes Observables, resulting in a component
+ * that safely subscribes to its Observables before rendering. The component
+ * will return null until the subscriptions are created.
+ */
+export function subscribe<P>(render: FC<P>): FC<P> {
+  const InnerComponent: FC<{ p: P }> = ({ p }) => (
+    <RemoveSubscribe>{render(p)}</RemoveSubscribe>
+  );
+  const OuterComponent: FC<P> = (p) => (
+    <Subscribe>
+      <InnerComponent p={p} />
+    </Subscribe>
+  );
+  // Copy over the component's display name, default props, etc.
+  Object.assign(OuterComponent, render);
+  return OuterComponent;
+}