nav: Stop dispatching all actions via Redux.

Instead, dispatch them via our recently added `NavigationService`.

Add a few notes in `InitialNavigationDispatcher` about why we expect
`NavigationService` to be ready in time. We chose to do the initial
navigation here, rather than in a descendant of the
`NavigationService`'s `ref`fed component for those reasons [1].

[1] zulip#4274 (comment)

Author: chrisbobbe

src/message/fetchActions.js

@@ -1,7 +1,14 @@
 /* @flow strict-local */
 import NavigationService from '../nav/NavigationService';
-import type { Narrow, Dispatch, GetState, GlobalState, Message, Action } from '../types';
-import type { ApiResponseServerSettings } from '../api/settings/getServerSettings';
+import type {
+  Narrow,
+  Dispatch,
+  GetState,
+  GlobalState,
+  Message,
+  Action,
+  ApiResponseServerSettings,
+} from '../types';
 import type { InitialData } from '../api/initialDataTypes';
 import * as api from '../api';
 import { resetToMainTabs } from '../actions';