Modal: remove support for onFinished callback (#29852)

* Fix up type for `finished` result of Modal

The `finished` promise can be called with an empty array, for example if the
dialog is closed by a background click. This was not correctly represented in
the typing. Fix that, and add some documentation while we're at it.

* Type fixes to onFinished callbacks from Modal

These can all be called with zero arguments, despite what the type annotations
may say, so mark them accordingly.

* Remove uses of Modal `onFinished` property

... because it is confusing.

Instead, use the `finished` promise returned by `createDialog`.

* Modal: remove support for now-unused `onFinished` prop

* StopGapWidgetDriver: use `await` instead of promise chaining

* Fix up unit tests

src/components/structures/RoomView.tsx

@@ -1753,7 +1753,7 @@ export class RoomView extends React.Component<IRoomProps, IRoomState> {
         }
 
         if (reportRoom !== false) {
-            actions.push(this.context.client.reportRoom(this.state.room.roomId, reportRoom));
+            actions.push(this.context.client.reportRoom(this.state.room.roomId, reportRoom!));
         }
 
         actions.push(this.context.client.leave(this.state.room.roomId));