Mainnet readiness

[j-mueller]

1. Add a type DemoEnvironment with all the values that we need to parameterise. Eg. blockfrost URL, seed phrases, explorer URL and so on
2. Add a route on the backend to return the DemoEnvironment. This will serve a JSON file with the mainnet parameters. That way we don't need to check them into the repository
3. Add a CLI option --demo-environment. This can be set to a JSON file with the DemoEnvironment that we should use. If no value is provided then it will default to the existing deployment on the preview network.
4. Add a react context with the demo environment and load it when the page loads.
5. Use values from the demo environment for all requests, wallet seed phrases and so forth.

[choener]

• All on cardano testnet
• I can't unfreeze 'Alice'
• Actually, any operation fails now, with either e.selectWallet or e.utxosAtWithUnit (frontend, browser console)
• Both WST and Ada are shown
• (running mainnet compiled frontend code and main backend works)