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.
note: I find "chains" less ambiguous than "networks" or "net".
Though the codebase already uses "net"/"network" for the same meaning,
e.g. see `electrum.constants.net` or the `getinfo` cmd dict containing a "network" key.. :/
