Don't create a guest login if user went to /login

This fixes an unintuitive behaviour where, if you follow a link to
riot.im/app/#/login, we take you to the login page, but not before we've
registered a guest account (or restarted the MatrixClient with the stored
creds).

This actually ends up simplifying some of the startup dance, as we special-case
the registration flows earlier on.

src/Lifecycle.js

@@ -35,9 +35,6 @@ import { _t } from './languageHandler';
  * Called at startup, to attempt to build a logged-in Matrix session. It tries
  * a number of things:
  *
- * 0. if it looks like we are in the middle of a registration process, it does
- *    nothing.
- *
  * 1. if we have a loginToken in the (real) query params, it uses that to log
  *    in.
  *
@@ -80,14 +77,6 @@ export function loadSession(opts) {
     const guestIsUrl = opts.guestIsUrl;
     const defaultDeviceDisplayName = opts.defaultDeviceDisplayName;
 
-    if (fragmentQueryParams.client_secret && fragmentQueryParams.sid) {
-        // this happens during email validation: the email contains a link to the
-        // IS, which in turn redirects back to vector. We let MatrixChat create a
-        // Registration component which completes the next stage of registration.
-        console.log("Not registering as guest: registration already in progress.");
-        return q();
-    }
-
     if (!guestHsUrl) {
         console.warn("Cannot enable guest access: can't determine HS URL to use");
         enableGuest = false;

src/components/structures/MatrixChat.js

@@ -269,6 +269,21 @@ module.exports = React.createClass({
             Lifecycle.initRtsClient(this.props.config.teamServerConfig.teamServerURL);
         }
 
+        // if the user has followed a login or register link, don't reanimate
+        // the old creds, but rather go straight to the relevant page
+
+        const firstScreen = this.state.screenAfterLogin ?
+            this.state.screenAfterLogin.screen : null;
+
+        if (firstScreen === 'login' ||
+                firstScreen === 'register' ||
+                firstScreen === 'forgot_password') {
+            this.props.onLoadCompleted();
+            this.setState({loading: false});
+            this._showScreenAfterLogin();
+            return;
+        }
+
         // the extra q() ensures that synchronous exceptions hit the same codepath as
         // asynchronous ones.
         q().then(() => {
@@ -840,14 +855,6 @@ module.exports = React.createClass({
     _onLoadCompleted: function() {
         this.props.onLoadCompleted();
         this.setState({loading: false});
-
-        // Show screens (like 'register') that need to be shown without _onLoggedIn
-        // being called. 'register' needs to be routed here when the email confirmation
-        // link is clicked on.
-        if (this.state.screenAfterLogin &&
-            ['register'].indexOf(this.state.screenAfterLogin.screen) !== -1) {
-            this._showScreenAfterLogin();
-        }
     },
 
     /**
@@ -946,6 +953,7 @@ module.exports = React.createClass({
                 this.state.screenAfterLogin.screen,
                 this.state.screenAfterLogin.params,
             );
+            // XXX: is this necessary? `showScreen` should do it for us.
             this.notifyNewScreen(this.state.screenAfterLogin.screen);
             this.setState({screenAfterLogin: null});
         } else if (localStorage && localStorage.getItem('mx_last_room_id')) {