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, at the dispatch sites.

Author: chrisbobbe

src/message/fetchActions.js

@@ -10,7 +10,7 @@ import type {
 } from '../types';
 import type { InitialData } from '../api/initialDataTypes';
 import * as api from '../api';
-import { resetToAccountPicker } from '../actions';
+import { resetToAccountPicker, resetToMainTabs } from '../actions';
 import { isClientError } from '../api/apiErrors';
 import {
   getAuth,
@@ -19,6 +19,7 @@ import {
   getLastMessageId,
   getCaughtUpForNarrow,
   getFetchingForNarrow,
+  getCurrentRouteName,
 } from '../selectors';
 import config from '../config';
 import {
@@ -323,6 +324,9 @@ export const doInitialFetch = () => async (dispatch: Dispatch, getState: GetStat
   }
 
   dispatch(realmInit(initData, new ZulipVersion(serverSettings.zulip_version)));
+  if (getCurrentRouteName() !== 'main') {
+    dispatch(resetToMainTabs());
+  }
   dispatch(initialFetchComplete());
   dispatch(startEventPolling(initData.queue_id, initData.last_event_id));
 

src/nav/__tests__/navReducer-test.js

@@ -2,9 +2,7 @@
 import type { NavigationAction } from 'react-navigation';
 
 import type { NavigationState, Action } from '../types';
-import { getCurrentRouteName } from './navSelectors';
 import AppNavigator from './AppNavigator';
-import { INITIAL_FETCH_COMPLETE } from '../actionConstants';
 
 /**
  * Get the initial state for the given route.
@@ -29,9 +27,6 @@ export const initialState = getStateForRoute('loading');
 
 export default (state: NavigationState = initialState, action: Action): NavigationState => {
   switch (action.type) {
-    case INITIAL_FETCH_COMPLETE:
-      return getCurrentRouteName() === 'main' ? state : getStateForRoute('main');
-
     default: {
       // The `react-navigation` libdef says this only takes a NavigationAction,
       // but docs say pass arbitrary action. $FlowFixMe