Hi @glowkeeper
The Simple ERC777 token example shows how to deploy ERC1820 for a local testnet (e.g. ganache-cli). Let me know if something is missing or needs more detail and I can update the example.
ERC1820 is deployed to all public networks (https://github.com/0xjac/ERC1820#erc1820-registry) so you don’t need to deploy it for public testnets such as Rinkeby or Ropsten. You only need to deploy it to local testnets (e.g. ganache-cli) or private networks. (Your migrations script appears to try to deploy for Rinkeby or Ropsten).
ERC1820 is used in the hooks functionality of ERC777, see the documentation for more detail:
Let me know if you need more information on the why and I can try to dig a little deeper.