Adds the mutinynet signet (https://github.com/MutinyWallet/mutiny-net/)
to the supported networks so it's possible to
use it with Electrum. Mutinynet is similar to signet, except for a
shorter block interval (30 sec), a nice faucet
(faucet.mutinynet.com) and some extra features in its core fork.
It also has some lightning network nodes, see
the mempool instance on mutinynet.com.
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