navReducer: Transplant away ACCOUNT_SWITCH 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 (zulip#3804).

So, convert the `navReducer`'s `ACCOUNT_SWITCH` case into
instructions to navigate to the loading screen.

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

src/account/accountActions.js

@@ -7,14 +7,19 @@ import {
   LOGIN_SUCCESS,
   LOGOUT,
 } from '../actionConstants';
-import { resetToAccountPicker } from '../nav/navActions';
+import { resetToAccountPicker, resetToLoading } from '../nav/navActions';
 import type { ZulipVersion } from '../utils/zulipVersion';
 
-export const accountSwitch = (index: number): Action => ({
+const accountSwitchPlain = (index: number): Action => ({
   type: ACCOUNT_SWITCH,
   index,
 });
 
+export const accountSwitch = (index: number) => (dispatch: Dispatch, getState: GetState) => {
+  dispatch(resetToLoading());
+  dispatch(accountSwitchPlain(index));
+};
+
 export const realmAdd = (
   realm: URL,
   zulipFeatureLevel: number,

src/nav/navReducer.js

@@ -3,7 +3,7 @@ import type { NavigationAction } from 'react-navigation';
 
 import type { NavigationState, Action } from '../types';
 import AppNavigator from './AppNavigator';
-import { INITIAL_FETCH_COMPLETE, ACCOUNT_SWITCH, LOGIN_SUCCESS } from '../actionConstants';
+import { INITIAL_FETCH_COMPLETE, LOGIN_SUCCESS } from '../actionConstants';
 
 /**
  * Get the initial state for the given route.
@@ -28,7 +28,6 @@ export const initialState = getStateForRoute('loading');
 
 export default (state: NavigationState = initialState, action: Action): NavigationState => {
   switch (action.type) {
-    case ACCOUNT_SWITCH:
     case LOGIN_SUCCESS:
       return getStateForRoute('loading');