Dynamic Badge Link

[lautarodragan]

Extracted from #43

Change the badge URL from https://explorer.po.et/works/ to either http://explorer-mainnet.poetnetwork.net/works/ or http://explorer-testnet.poetnetwork.net/works/, depending on whether the API Token does not have (mainnet) or has (testnet) "TEST_" at the beginning of it. This is because we are now supporting two different environments (LIVE and TEST) which write to the Bitcoin mainnet and testnet blockchains respectively. Fallback option here is to default it to the mainnet link since most WP users will be wanting to have their content anchored only to the mainnet blockchain anyway.