Implement admin container and pausable contracts

[Westlad]

fixes #718
fixes #675
This PR is quite big, but it's mainly additive code. It creates an administrator container from which one can call all of the 'onlyOwner' functions in the polygon-nightfall smart contracts (except for registering a new verifying key, which shouldn't be a manual process). Currently we have no simple way to call these functions. It also adds a 'pausable' interface to our critical contracts (State, Shield) so that these can be paused (frozen) in the event of an attack.
Testing is manual (this may change in future but there isn't time now) but easy:

1. Run up nightfall as normal (./start-nightfall -g -s -d). This will now also create an admin container.
2. Exec into this container. The easiest way is with docker-compose exec administrator bash.
3. Run the administrator application (./admin).
4. You will be asked for the owner private key. The easiest way to find this is to look in docker-compose.ganache.yml. The owner key is the first of the four listed there. You can use the others to test some of the functions that require private keys. The key will be remembered until you exit the ./admin application.
5. When you have finished, exit the application. It will then forget the private key. Note however that your changes will persist on the ganache blockchain, for example if you transfer ownership of nightfall, you'll need to use the new private key until you restart polygon-nightfall.
   Most of the actions will emit an event which will be picked up (and then ignored) by optimist so that you can tell if the call has worked. Note that client will see events emitted by the State contract but not the Shield contract because it doesn't listen to that.

[druiz0992]

You will need to rebase with master and failing tests should now pass