Make allow list manager settable.

[nlordell]

See discussion in #351

@fedgiac: Upgrading the proxy is a relatively expensive operations (as pointed out by frangio), and changing the manager is something that we might need to do from the start, for example because we want to assign this power to some other staking contract, or a DAO. Note that we use the same owner address on mainnet and Rinkeby for now: the first transaction I expect after releasing is changing the owner of the mainnet contract to some other address. Then also the manager should be changing.

[bh2smith]

Here is some code we had considered using that allows the proxyAdmin to update the manager

    modifier onlyProxyAdmin() {
        // Slot taken from https://eips.ethereum.org/EIPS/eip-1967#specification
        // obtained as bytes32(uint256(keccak256('eip1967.proxy.admin')) - 1)
        bytes32 slot =
            0xb53127684a568b3173ae13b9f8a6016e243e63b6e8ee1178d6a717850b5d6103;
        address proxyAdmin;
        // solhint-disable-next-line no-inline-assembly
        assembly {
            proxyAdmin := sload(slot)
        }
        require(msg.sender == proxyAdmin, "GPv2: caller not proxyAdmin");
        _;
    }

    function updateManager(address newManager) external onlyProxyAdmin {
        manager = newManager;
    }

[nlordell]

Implemented by #407