[explorer] Show the miner name in the block page

Summary:
T3543

Parse coinbase string for miner. Display on block page.

Test Plan:

cd explorer-exe
cargo run

check rendering for a few blocks

cd explorer-server
cargo test

tests pass

Reviewers: #bitcoin_abc, tobias_ruck

Reviewed By: #bitcoin_abc, tobias_ruck

Subscribers: tobias_ruck, PiRK, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16533