navReducer: Transplant away LOGIN_SUCCESS 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 `LOGIN_SUCCESS` 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

@@ -36,13 +36,21 @@ export const removeAccount = (index: number): Action => ({
   index,
 });
 
-export const loginSuccess = (realm: URL, email: string, apiKey: string): Action => ({
+const loginSuccessPlain = (realm: URL, email: string, apiKey: string): Action => ({
   type: LOGIN_SUCCESS,
   realm,
   email,
   apiKey,
 });
 
+export const loginSuccess = (realm: URL, email: string, apiKey: string) => (
+  dispatch: Dispatch,
+  getState: GetState,
+) => {
+  dispatch(resetToLoading());
+  dispatch(loginSuccessPlain(realm, email, apiKey));
+};
+
 const logoutPlain = (): Action => ({
   type: LOGOUT,
 });

src/nav/__tests__/navReducer-test.js

@@ -1,32 +1,9 @@
 import deepFreeze from 'deep-freeze';
 
-import { LOGIN_SUCCESS, INITIAL_FETCH_COMPLETE } from '../../actionConstants';
+import { INITIAL_FETCH_COMPLETE } from '../../actionConstants';
 import navReducer, { getStateForRoute } from '../navReducer';
 
 describe('navReducer', () => {
-  describe('LOGIN_SUCCESS', () => {
-    test('replaces the existing route stack with "loading" on sign in', () => {
-      const prevState = deepFreeze({
-        index: 2,
-        routes: [{ key: 'one' }, { key: 'two' }, { key: 'password' }],
-      });
-
-      const action = deepFreeze({
-        type: LOGIN_SUCCESS,
-      });
-
-      const expectedState = {
-        index: 0,
-        routes: [{ routeName: 'loading' }],
-      };
-
-      const newState = navReducer(prevState, action);
-
-      expect(newState.index).toEqual(expectedState.index);
-      expect(newState.routes[0].routeName).toEqual(expectedState.routes[0].routeName);
-    });
-  });
-
   describe('INITIAL_FETCH_COMPLETE', () => {
     test('do not mutate navigation state if already at the same route', () => {
       const prevState = getStateForRoute('main');

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, LOGIN_SUCCESS } from '../actionConstants';
+import { INITIAL_FETCH_COMPLETE } from '../actionConstants';
 
 /**
  * Get the initial state for the given route.
@@ -28,9 +28,6 @@ export const initialState = getStateForRoute('loading');
 
 export default (state: NavigationState = initialState, action: Action): NavigationState => {
   switch (action.type) {
-    case LOGIN_SUCCESS:
-      return getStateForRoute('loading');
-
     case INITIAL_FETCH_COMPLETE:
       return state.routes[0].routeName === 'main' ? state : getStateForRoute('main');