Implement the new unified grid layout

Here I've implemented an MVP for the new unified grid layout, which scales smoothly up to arbitrarily many participants. It doesn't yet have a special 1:1 layout, so in spotlight mode and 1:1s, we will still fall back to the legacy grid systems.

Things that happened along the way:
- The part of VideoTile that is common to both spotlight and grid tiles, I refactored into MediaView
- VideoTile renamed to GridTile
- Added SpotlightTile for the new, glassy spotlight designs
- NewVideoGrid renamed to Grid, and refactored to be even more generic
- I extracted the media name logic into a custom React hook
- Deleted the BigGrid experiment

src/useReactiveState.ts

@@ -44,7 +44,7 @@ export const useReactiveState = <T>(
   if (
     prevDeps.current === undefined ||
     deps.length !== prevDeps.current.length ||
-    deps.some((d, i) => d !== prevDeps.current![i])
+    deps.some((d, i) => !Object.is(d, prevDeps.current![i]))
   ) {
     state.current = updateFn(state.current);
   }