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.tsx

@@ -15,7 +15,7 @@ limitations under the License.
 */
 
 import classNames from "classnames";
-import { FC, HTMLAttributes, ReactNode } from "react";
+import { FC, HTMLAttributes, ReactNode, forwardRef } from "react";
 import { Link } from "react-router-dom";
 import { useTranslation } from "react-i18next";
 import { Heading, Text } from "@vector-im/compound-web";
@@ -32,13 +32,21 @@ interface HeaderProps extends HTMLAttributes<HTMLElement> {
   className?: string;
 }
 
-export const Header: FC<HeaderProps> = ({ children, className, ...rest }) => {
-  return (
-    <header className={classNames(styles.header, className)} {...rest}>
-      {children}
-    </header>
-  );
-};
+export const Header = forwardRef<HTMLElement, HeaderProps>(
+  ({ children, className, ...rest }, ref) => {
+    return (
+      <header
+        ref={ref}
+        className={classNames(styles.header, className)}
+        {...rest}
+      >
+        {children}
+      </header>
+    );
+  },
+);
+
+Header.displayName = "Header";
 
 interface LeftNavProps extends HTMLAttributes<HTMLElement> {
   children: ReactNode;