Add componentized support for testnet, alternate chains

[bedinotti]

Testnet verification is enabled in the web component, but we've been reluctant to add the same support in our mobile apps, for fear that an attacker would just generate a (valid) testnet certificate and pass it off as a main net one.

We'd also like to add support for alternate blockchains (Ethereum, for example).

It would be great if we could allow these alternate chains to be toggled for validation support in settings. This would mean that, by default, we could only allow mainnet on one (or multiple) chains, but let advanced users or developers perform full verification on testnet certificates.