Add links to mempool.space

[casey]

I often want to look up blocks, transactions, and outputs on mempool.space. We should include links on those pages to make it easier. Will have to take signet/mainnet/testnet into account.

[batcavekid]

Yes pls. Was going to make same issue.

[Psifour]

Relevant http endpoints and an icon to add to the right of the block, tx, and outputs links

https://mempool.space/block/*blocknumber* https://mempool.space/block/*blockhash* https://mempool.space/tx/*transactionhash* https://mempool.space/address/*address*