Give autocomplete providers the room they're in

Removes the gut-wrenching that RoomView does to jam the user list
into the user autocomplete provider.

src/autocomplete/AutocompleteProvider.js

@@ -28,6 +28,9 @@ export default class AutocompleteProvider {
         }
     }
 
+    destroy() {
+    }
+
     /**
      * Of the matched commands in the query, returns the first that contains or is contained by the selection, or null.
      */