Log out of the guest account if coming from the welcome page

This is ultimately the fix for element-hq/element-web#9224 however it is not really great. By not logging out of the guest account, getCurrentHsUrl() returns the wrong homeserver URL (matrix.org) instead of the discovered one.

src/Lifecycle.js

@@ -415,16 +415,18 @@ export function logout() {
         // Also we sometimes want to re-log in a guest session
         // if we abort the login
 
-        // use settimeout to avoid racing with react unmounting components
-        // which need a valid matrixclientpeg
-        setTimeout(()=>{
-            onLoggedOut();
-        }, 0);
-        return;
+        return new Promise((resolve, _reject) => {
+            // use settimeout to avoid racing with react unmounting components
+            // which need a valid matrixclientpeg
+            setTimeout(()=>{
+                onLoggedOut();
+                resolve();
+            }, 0);
+        });
     }
 
     _isLoggingOut = true;
-    MatrixClientPeg.get().logout().then(onLoggedOut,
+    return MatrixClientPeg.get().logout().then(onLoggedOut,
         (err) => {
             // Just throwing an error here is going to be very unhelpful
             // if you're trying to log out because your server's down and

src/components/structures/MatrixChat.js

@@ -502,15 +502,26 @@ export default React.createClass({
                 startAnyRegistrationFlow(payload);
                 break;
             case 'start_registration':
-                // This starts the full registration flow
-                this._startRegistration(payload.params || {});
-                break;
-            case 'start_login':
-                this.setStateForNewView({
-                    view: VIEWS.LOGIN,
+            case 'start_login': {
+                let logoutPromise = Promise.resolve();
+                if (payload.params && payload.params.logoutFirst) {
+                    logoutPromise = Lifecycle.logout();
+                }
+                logoutPromise.then(() => {
+                    if (payload.action === "start_login") {
+                        this.setStateForNewView({
+                            view: VIEWS.LOGIN,
+                        });
+                        this.notifyNewScreen('login');
+                    } else if (payload.action === "start_registration") {
+                        // This starts the full registration flow
+                        this._startRegistration(payload.params || {});
+                    } else {
+                        throw new Error("Unknown action for post-logout");
+                    }
                 });
-                this.notifyNewScreen('login');
                 break;
+            }
             case 'start_post_registration':
                 this.setState({
                     view: VIEWS.POST_REGISTRATION,