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

public/locales/en-GB/app.json

@@ -41,6 +41,7 @@
     "analytics": "Analytics",
     "audio": "Audio",
     "avatar": "Avatar",
+    "back": "Back",
     "camera": "Camera",
     "copied": "Copied!",
     "display_name": "Display name",
@@ -49,6 +50,7 @@
     "home": "Home",
     "loading": "Loading…",
     "microphone": "Microphone",
+    "next": "Next",
     "options": "Options",
     "password": "Password",
     "profile": "Profile",