[plugin-architecture] Handle Mainnet/Testnet envs

[victorgcramos]

Our /api endpoint returns whether the app is running on mainnet or testnet, and we should handle those environments on our politeia app-shell.

We should handle the following stuff:

• Dcrdata links: block height, transactions and voting charts links
• Block params: block duration
• Voting requirements: min/max quorum and pass percentages