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/grid/GridLayout.module.css

@@ -0,0 +1,54 @@
+/*
+Copyright 2024 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.
+*/
+
+.scrolling {
+  box-sizing: border-box;
+  block-size: 100%;
+  display: flex;
+  flex-wrap: wrap;
+  justify-content: center;
+  align-content: center;
+  gap: var(--gap);
+  box-sizing: border-box;
+}
+
+.scrolling > .slot {
+  width: var(--width);
+  height: var(--height);
+}
+
+.fixed > .slot {
+  position: absolute;
+  inline-size: 404px;
+  block-size: 233px;
+  inset: -12px;
+}
+
+.fixed > .slot[data-block-alignment="start"] {
+  inset-block-end: unset;
+}
+
+.fixed > .slot[data-block-alignment="end"] {
+  inset-block-start: unset;
+}
+
+.fixed > .slot[data-inline-alignment="start"] {
+  inset-inline-end: unset;
+}
+
+.fixed > .slot[data-inline-alignment="end"] {
+  inset-inline-start: unset;
+}