Add redundancy to blockchain validation requests

[bedinotti]

Right now, we only one sone blockchain explorer (https://blockchain.info) to validate that a certificate is in fact, on the blockchain. This creates a hard dependency where if (for whatever reason) that endpoint is unresponsive, returning bad data, or is just unreachable, then we'll state that valid certificates are in fact invalid.

It would be good to:

1. Have a list of these blockchain explorers so we can still validate if one goes down.
2. Maybe provide some nuance to validation? If we get an invalid response, say "try again" or "unsure" vs calling it "invalid"