Implement most of the remaining layout changes

Includes the mobile UX optimizations and the tweaks we've made to cut down on wasted space, but does not yet include the change to embed the spotlight tile within the grid.

src/observable-utils.ts

@@ -14,7 +14,7 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
-import { Observable, defer, finalize, tap } from "rxjs";
+import { Observable, defer, finalize, scan, startWith, tap } from "rxjs";
 
 const nothing = Symbol("nothing");
 
@@ -35,3 +35,15 @@ export function finalizeValue<T>(callback: (finalValue: T) => void) {
       );
     });
 }
+
+/**
+ * RxJS operator that accumulates a state from a source of events. This is like
+ * scan, except it emits an initial value immediately before any events arrive.
+ */
+export function accumulate<State, Event>(
+  initial: State,
+  update: (state: State, event: Event) => State,
+) {
+  return (events: Observable<Event>): Observable<State> =>
+    events.pipe(scan(update, initial), startWith(initial));
+}