hidden email: Smooth transitions with a bit of fake blank data.

We're about to start using this `email` property for `getOwnEmail`;
and we'd like to have it throw an error if we don't actually have a
valid own-email because there isn't a logged-in active account.

Unfortunately it turns out -- as we learn from trying that -- that
there are existing parts of our code that can end up trying to get at
the own-email when there is no valid answer.  That's a bug, but for
now just keep it papered over by making up some blank data (like we do
in a lot of our legacy code), and make a TODO comment.

The harder-line `initialState` value still has a foothold, in that we
use it on first startup.

src/realm/__tests__/realmReducer-test.js

@@ -11,7 +11,7 @@ import { NULL_OBJECT } from '../../nullObjects';
 
 describe('realmReducer', () => {
   describe('ACCOUNT_SWITCH', () => {
-    test('resets state to initial state', () => {
+    test('resets state to blank state', () => {
       const initialState = NULL_OBJECT;
 
       const action = deepFreeze({
@@ -21,6 +21,7 @@ describe('realmReducer', () => {
       const expectedState = {
         canCreateStreams: true,
         crossRealmBots: [],
+        email: '',
         isAdmin: false,
         twentyFourHourTime: false,
         emoji: {},

src/realm/realmReducer.js

@@ -22,6 +22,23 @@ const initialState = {
   nonActiveUsers: [],
 };
 
+/**
+ * A version of `initialState` with some made-up blank data.
+ *
+ * On `LOGIN_SUCCESS`, we go straight to showing the main app UI (see
+ * `navReducer`) even though we're still loading the actual data from the
+ * server.  So we need some fake data that the UI code will swallow.
+ * TODO: Probably stop doing that.
+ *
+ * Also: On `ACCOUNT_SWITCH`, during the transition animation, some old
+ * components can still be mounted from the UI for the previous account that
+ * make no sense without server data.  Probably ditto `LOGOUT`.
+ */
+const fakeBlankState = {
+  ...initialState,
+  email: '',
+};
+
 const convertRealmEmoji = (data): RealmEmojiById => {
   const emojis = {};
   Object.keys(data).forEach(id => {
@@ -35,7 +52,7 @@ export default (state: RealmState = initialState, action: Action): RealmState =>
     case LOGOUT:
     case LOGIN_SUCCESS:
     case ACCOUNT_SWITCH:
-      return initialState;
+      return fakeBlankState;
 
     case REALM_INIT: {
       return {