Add channel to group when connecting

See also: #17

wlm_server/operation/consumers.py

@@ -0,0 +1,16 @@
+from channels.generic.websocket import AsyncWebsocketConsumer
+
+class OperationConsumer(AsyncWebsocketConsumer):
+    """Consumer for notifying the operation change of a specific channel.
+    
+    Attributes:
+        ch: Target WLM channel.
+        group_name: Name of group it belongs to in the channel layer.
+    """
+
+    # pylint: disable=attribute-defined-outside-init
+    async def connect(self):
+        self.ch = self.scope['url_route']['kwargs']['ch']
+        self.group_name = f'channel_{self.ch}'
+        await self.channel_layer.group_add(self.group_name, self.channel_name)
+        await self.accept()