Fix bug where email was not required where it shouldn't have been

See comment!

Fixes https://github.com/vector-im/riot-web/issues/9681

src/components/views/auth/RegistrationForm.js

@@ -1,7 +1,7 @@
 /*
 Copyright 2015, 2016 OpenMarket Ltd
 Copyright 2017 Vector Creations Ltd
-Copyright 2018 New Vector Ltd
+Copyright 2018, 2019 New Vector Ltd
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.
@@ -150,7 +150,11 @@ module.exports = React.createClass({
             if (!field) {
                 continue;
             }
-            field.validate({ allowEmpty: false });
+            // We must wait for these validations to finish before queueing
+            // up the setState below so our setState gies in the queue after
+            // all the setStates from these validate calls (that's how we
+            // know they've finished).
+            await field.validate({ allowEmpty: false });
         }
 
         // Validation and state updates are async, so we need to wait for them to complete