Convert view_user dispatch to prove the conversion works

This is a relatively obvious dispatch action that doesn't require a lot of complicated type definitions, so should be a good candidate to prove the thing works. If for some reason the thing stops working, we've done something wrong.

This also adds a bit of generic types to the dispatch call so we don't confuse the tsx parser by using `dis.dispatch(<ViewUserPayload>{...})` as it thinks that's supposed to be a component. We still get type safety, and the thing remains happy with the generics approach.

src/dispatcher/dispatcher.ts

@@ -32,7 +32,7 @@ export class MatrixDispatcher extends Dispatcher<ActionPayload> {
      *        an operation that the browser requires user interaction
      *        for. Default false (async).
      */
-    dispatch(payload: ActionPayload, sync = false) {
+    dispatch<T extends ActionPayload>(payload: T, sync = false) {
         if (payload instanceof AsyncActionPayload) {
             payload.fn((action: ActionPayload) => {
                 this.dispatch(action, sync);