notification_data: Annotate flags with Collection.

Since `flags` here could be iterated through multiple times
(to check for push/email notifiability), we use `Collection`.
Inspired by 871e73ab8f.

The other change here in the `event_queue` code is prep for using
the `UserMessageNotificationsData` class there.

zerver/lib/notification_data.py

@@ -1,11 +1,11 @@
 from dataclasses import dataclass
-from typing import List
+from typing import Collection
 
 
 @dataclass
 class UserMessageNotificationsData:
     id: int
-    flags: List[str]
+    flags: Collection[str]
     mentioned: bool
     online_push_enabled: bool
     stream_push_notify: bool