message send: Introduce dataclass to wrap user-notifications variables.

We will in later commits, extend this class to contain methods
to determine if a message is notifiable or not, but for now
we only turn it into a dict and pass it on.

zerver/lib/notification_data.py

@@ -0,0 +1,20 @@
+from dataclasses import dataclass
+from typing import List
+
+
+@dataclass
+class UserMessageNotificationsData:
+    id: int
+    flags: List[str]
+    mentioned: bool
+    online_push_enabled: bool
+    stream_push_notify: bool
+    stream_email_notify: bool
+    wildcard_mention_notify: bool
+    sender_is_muted: bool
+
+    def __post_init__(self) -> None:
+        if self.mentioned:
+            assert "mentioned" in self.flags
+        if self.wildcard_mention_notify:
+            assert "wildcard_mentioned" in self.flags