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.

.eslintrc.cjs

@@ -38,6 +38,15 @@ module.exports = {
     "jsx-a11y/media-has-caption": "off",
     // We should use the js-sdk logger, never console directly.
     "no-console": ["error"],
+    "no-restricted-imports": [
+      "error",
+      {
+        name: "@react-rxjs/core",
+        importNames: ["Subscribe", "RemoveSubscribe"],
+        message:
+          "These components are easy to misuse, please use the 'subscribe' component wrapper instead",
+      },
+    ],
   },
   settings: {
     react: {