InitialNavigationDispatcher: Clarify and improve doInitialNavigation.

When the active account isn't logged in and there's just one account
in total, we can reasonably pre-fill that account's realm in the
realm-input screen. It seems like this has been the expected
behavior [1], but I just tested it now, and it hasn't been
happening.

Also, for readability, separate the `accounts.length === 1` check
into its own `else if`.

[1] https://chat.zulip.org/#narrow/stream/243-mobile-team/topic/decouple.20nav.20from.20redux.20%28.23M3804%29/near/1042602

src/nav/InitialNavigationDispatcher.js

@@ -43,12 +43,26 @@ class InitialNavigationDispatcher extends PureComponent<Props> {
   doInitialNavigation = () => {
     const { hasAuth, accounts, haveServerData, dispatch } = this.props;
 
-    // If the active account is not logged in, show account screen.
+    // If the active account is not logged in, bring the user as close
+    // as we can to AuthScreen, the place where they can log in.
     if (!hasAuth) {
       if (accounts.length > 1) {
+        // We can't guess which account, of multiple, the user wants
+        // to use. Let them pick one.
         dispatch(resetToAccountPicker());
         return;
+      } else if (accounts.length === 1) {
+        // We already know the realm, so give that to the realm
+        // screen. If that screen finds that the realm is valid, it'll
+        // send the user along to AuthScreen for that realm right
+        // away. If this means you're on the AuthScreen when you don't
+        // want to be (i.e., you want to choose a different realm),
+        // you can always go back to RealmScreen.
+        dispatch(resetToRealmScreen({ initial: true, realm: accounts[0].realm }));
+        return;
       } else {
+        // Just go to the realm screen and have the user type out the
+        // realm.
         dispatch(resetToRealmScreen({ initial: true }));
         return;
       }