libdefs: Hard-code redux-thunk support into redux-mock-store.

As foreshadowed in the preceding commits, we can't really expect a
FlowTyped-hosted libdef to have the flexibility to give us the right
types for Dispatch on the condition that we pass `thunk` as a piece
of middleware.

We're decidedly using `thunk`, so, hard-code support for it here by
allowing dispatch to be called with a redux-thunk async action.

We're pretty consistent with having correct types for such an
action, so not much is lost by typing its arguments loosely as
`Function, Function`, and from some attempts, it seems much easier
than nailing down something exactly right.

[cherry-picked from zulip#4171]

flow-typed/redux-mock-store_v1.2.x.js

@@ -5,12 +5,15 @@ declare module 'redux-mock-store' {
   */
 
   declare type mockStore = { <S, A>(state: S): mockStoreWithoutMiddleware<S, A>, ... };
-  declare type DispatchAPI<A> = (action: A) => A;
-  declare type Dispatch<A: $ReadOnly<{ type: string, ... }>> = DispatchAPI<A>;
+  declare interface Dispatch<S, A> {
+    (action: A): A;
+    <T>((Function, Function) => T): T;
+  }
+
   declare type mockStoreWithoutMiddleware<S, A> = {
     getState(): S,
     getActions(): Array<A>,
-    dispatch: Dispatch<A>,
+    dispatch: Dispatch<S, A>,
     clearActions(): void,
     subscribe(callback: () => void): () => void,
     replaceReducer(nextReducer: Function): void,