contants: add support for mutinynet signet

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.

electrum/chains/mutinynet/fallback_lnnodes.json

@@ -0,0 +1,18 @@
+{
+  "02465ed5be53d04fde66c9418ff14a5f2267723810176c9212b722e542dc1afb1b": {
+    "host": "45.79.52.207",
+    "port": 9735
+  },
+  "032ae843e4d7d177f151d021ac8044b0636ec72b1ce3ffcde5c04748db2517ab03": {
+    "host": "45.79.201.241",
+    "port": 9735
+  },
+  "0220566172d9e324b41ec6f74ca44d377d3faf72ddb310fd263e6d5bcde4882492": {
+    "host": "185.90.61.24",
+    "port": 9735
+  },
+  "035a4e767bb1be29ed20219b40f07d9be03656a5f83485821878963c05290a877c": {
+    "host": "54.158.203.78",
+    "port": 9746
+  }
+}

electrum/chains/mutinynet/servers.json

@@ -0,0 +1,7 @@
+{
+    "5.9.83.108": {
+        "pruning": "-",
+        "s": "51234",
+        "version": "1.4"
+    }
+}