Implement tracking of referrals (#659)

* Implement tracking of referrals

This also modifies (or fixes) auto-joining.

src/components/structures/MatrixChat.js

@@ -1055,6 +1055,7 @@ module.exports = React.createClass({
                     sessionId={this.state.register_session_id}
                     idSid={this.state.register_id_sid}
                     email={this.props.startingFragmentQueryParams.email}
+                    referrer={this.props.startingFragmentQueryParams.referrer}
                     username={this.state.upgradeUsername}
                     guestAccessToken={this.state.guestAccessToken}
                     defaultHsUrl={this.getDefaultHsUrl()}