Implement renumeration of ordered tags upon collision

I was being bitten by this enough for me to want to fix it. This implementation really ought to be improved such that it doesnt tend towards being broken the more it is used.

src/components/structures/RoomSubList.js

@@ -360,7 +360,13 @@ var RoomSubList = React.createClass({
 
         if (order === orderA || order === orderB) {
             console.error("Cannot describe new list position.  This should be incredibly unlikely.");
-            // TODO: renumber the list
+            this.state.sortedList.forEach((room, index) => {
+                MatrixClientPeg.get().setRoomTag(
+                    room.roomId, this.props.tagName,
+                    {order: index / this.state.sortedList.length},
+                );
+            });
+            return index / this.state.sortedList.length;
         }
 
         return order;