Ensure incremental updates to the ImportanceAlgorithm trigger A-Z order

Fixes https://github.com/vector-im/riot-web/issues/14475

Background: Sticky rooms are actually a pair of lies to the underlying algorithm as a combination of REMOVE_ROOM/NEW_ROOM calls so they don't get considered as needing to be sorted. When a room is added under the importance algorithm, it is expected that the category it is being added to will be re-sorted to account for the change, however we weren't doing that since we optimized the NewRoom path to be a splice operation.

src/stores/room-list/algorithms/list-ordering/ImportanceAlgorithm.ts

@@ -123,6 +123,7 @@ export class ImportanceAlgorithm extends OrderingAlgorithm {
             const category = this.getRoomCategory(room);
             this.alterCategoryPositionBy(category, 1, this.indices);
             this.cachedOrderedRooms.splice(this.indices[category], 0, room); // splice in the new room (pre-adjusted)
+            await this.sortCategory(category);
         } else if (cause === RoomUpdateCause.RoomRemoved) {
             const roomIdx = this.getRoomIndex(room);
             if (roomIdx === -1) {