Add state loggingIn to MatrixChat to fix flashing login

To prevent the login screen from flashing when refreshing the app, use some state to indicate that a login is in progress, and OR that with the existing `loading` boolean to show the `<Spinner>` instead of the default `<Login>`.

This might be too invasive, and a default spinner may be better.

src/Lifecycle.js

@@ -276,6 +276,11 @@ export function setLoggedIn(credentials) {
     console.log("setLoggedIn => %s (guest=%s) hs=%s",
                 credentials.userId, credentials.guest,
                 credentials.homeserverUrl);
+    // This is dispatched to indicate that the user is still in the process of logging in
+    // because `teamPromise` may take some time to resolve, breaking the assumption that
+    // `setLoggedIn` takes an "instant" to complete, and dispatch `on_logged_in` a few ms
+    // later than MatrixChat might assume.
+    dis.dispatch({action: 'on_logging_in'});
 
     // Resolves by default
     let teamPromise = Promise.resolve(null);

src/components/structures/MatrixChat.js

@@ -568,6 +568,9 @@ module.exports = React.createClass({
             case 'set_theme':
                 this._onSetTheme(payload.value);
                 break;
+            case 'on_logging_in':
+                this.setState({loggingIn: true});
+                break;
             case 'on_logged_in':
                 this._onLoggedIn(payload.teamToken);
                 break;
@@ -757,6 +760,7 @@ module.exports = React.createClass({
         this.setState({
             guestCreds: null,
             logged_in: true,
+            loggingIn: false,
         });
 
         if (teamToken) {
@@ -1160,7 +1164,11 @@ module.exports = React.createClass({
         // console.log("rendering; loading="+this.state.loading+"; screen="+this.state.screen +
         //             "; logged_in="+this.state.logged_in+"; ready="+this.state.ready);
 
-        if (this.state.loading) {
+        // `loading` might be set to false before `logged_in = true`, causing the default
+        // (`<Login>`) to be visible for a few MS (say, whilst a request is in-flight to
+        // the RTS). So in the meantime, use `loggingIn`, which is true between
+        // actions `on_logging_in` and `on_logged_in`.
+        if (this.state.loading || this.state.loggingIn) {
             var Spinner = sdk.getComponent('elements.Spinner');
             return (
                 <div className="mx_MatrixChat_splash">