navReducer: Transplant away INITIAL_FETCH_COMPLETE logic.

Soon, we'll dismantle `navReducer`, as its purpose is incompatible
with a design where we manage navigation state separately from our
Redux-stored data (#3804).

So, convert the `navReducer`'s `INITIAL_FETCH_COMPLETE` case into
instructions to navigate to the main-tabs screen, if not already
there.

Make the action creator into a thunk action creator and dispatch the
navigation action from there, to reduce repetition.

Author: chrisbobbe

src/message/fetchActions.js

@@ -10,6 +10,7 @@ import type {
 } from '../types';
 import type { InitialData } from '../api/initialDataTypes';
 import * as api from '../api';
+import { resetToMainTabs } from '../actions';
 import { isClientError } from '../api/apiErrors';
 import {
   getAuth,
@@ -18,6 +19,7 @@ import {
   getLastMessageId,
   getCaughtUpForNarrow,
   getFetchingForNarrow,
+  getNavigationRoutes,
 } from '../selectors';
 import config from '../config';
 import {
@@ -156,10 +158,20 @@ const initialFetchStart = (): Action => ({
   type: INITIAL_FETCH_START,
 });
 
-const initialFetchComplete = (): Action => ({
+const initialFetchCompletePlain = (): Action => ({
   type: INITIAL_FETCH_COMPLETE,
 });
 
+export const initialFetchComplete = () => async (dispatch: Dispatch, getState: GetState) => {
+  if (getNavigationRoutes()[0] !== 'main') {
+    // If we're anywhere in the normal UI of the app, then remain
+    // where we are. Only reset the nav state if we're elsewhere,
+    // and in that case, go to the main screen.
+    dispatch(resetToMainTabs());
+  }
+  dispatch(initialFetchCompletePlain());
+};
+
 /** Private; exported only for tests. */
 export const isFetchNeededAtAnchor = (
   state: GlobalState,

src/nav/__tests__/navReducer-test.js

@@ -2,9 +2,7 @@
 import type { NavigationAction } from 'react-navigation';
 
 import type { NavigationState, Action } from '../types';
-import { getNavigationRoutes } from './navSelectors';
 import AppNavigator from './AppNavigator';
-import { INITIAL_FETCH_COMPLETE } from '../actionConstants';
 
 /**
  * Get the initial state for the given route.
@@ -29,12 +27,6 @@ export const initialState = getStateForRoute('loading');
 
 export default (state: NavigationState = initialState, action: Action): NavigationState => {
   switch (action.type) {
-    case INITIAL_FETCH_COMPLETE:
-      // If we're anywhere in the normal UI of the app, then remain
-      // where we are. Only reset the nav state if we're elsewhere,
-      // and in that case, go to the main screen.
-      return getNavigationRoutes()[0].routeName === 'main' ? state : getStateForRoute('main');
-
     default: {
       // The `react-navigation` libdef says this only takes a NavigationAction,
       // but docs say pass arbitrary action. $FlowFixMe