Move configuration from config.py to config.json and add support for GitHub organization webhooks

remove _format_watch() from utils.github_webhook as the value of
watchers_count is always identical with stargazers_count
https://docs.github.com/en/developers/webhooks-and-events/webhook-events-and-payloads#watch
https://developer.github.com/changes/2012-09-05-watcher-api/

config_sample.json

@@ -0,0 +1,18 @@
+{
+  "bot_token": "123456:ABC-DEF1234ghIkl-zyx57W2v1u123ew11",
+  "port": 12345,
+  "gh_webhooks": {
+    "Codertocat/Hello-World": {
+      "chat_id": -1001234567890,
+      "secret": "FPAh9pwRHCLpRL7j"
+    },
+    "Octocoders/Hello-World": {
+      "chat_id": "@username",
+      "secret": "H4xvfPNCnUhPTERq"
+    },
+    "octo-org": {
+      "chat_id": "@username",
+      "secret": "KLrYeiA3vNLPVbAv"
+    }
+  }
+}