Remember we were trying to accept an invite

When the user was on an invite page and clicked the sign up/sign in
buttons, remember that invite so we can show it again after they're
done signing up/in.

https://github.com/vector-im/riot-web/issues/9816

src/components/structures/RoomView.js

@@ -2,6 +2,7 @@
 Copyright 2015, 2016 OpenMarket Ltd
 Copyright 2017 Vector Creations Ltd
 Copyright 2018, 2019 New Vector Ltd
+Copyright 2019 The Matrix.org Foundation C.I.C.
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.
@@ -1552,6 +1553,7 @@ module.exports = React.createClass({
                             inviterName={inviterName}
                             invitedEmail={invitedEmail}
                             oobData={this.props.oobData}
+                            signUrl={this.props.thirdPartyInvite.inviteSignUrl}
                             room={this.state.room}
                         />
                     </div>