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/Header.module.css

@@ -22,6 +22,7 @@ limitations under the License.
   user-select: none;
   flex-shrink: 0;
   padding-inline: var(--inline-content-inset);
+  padding-block-end: var(--cpd-space-4x);
 }
 
 .nav {