Move the legacy Rinkeby relayer to the new hub

[spalladino]

There is a relayer at rinkeby-01.gsn.openzeppelin.org/getaddr running on the old RelayHub version. We should switch it to the new hub.

Before doing that, make sure to migrate the chat app demo to the new hub as well, so it does not stop working.

[nventuro]

I deployed a new chat app on the new hub on 605c949, we still need to redeploy the site.

[nventuro]

The chat app is now working on the new RelayHub, but we shouldn't move the relayer from the old one to the new one: this is penalizable behavior (on both RelayHubs!).

What instead should happen is:

1. the relayer should deregister from the old hub
2. after the unstake delay, the relayer should retrieve its stake and send funds to its owner
3. a new relayer should be created and staked for on the new hub

We can of course drop 1. and 2. and just do 3.