long_term_idle_helper: Take timestamp_from_message callable arg.

message["ts"] is slack-specific. For this to be a general util function
it needs to take a callable that will grab a timestamp from the message
dict (which has varying formats depending on what we're importing from).

zerver/data_import/import_util.py

@@ -753,6 +753,7 @@ ExternalId = TypeVar("ExternalId")
 def long_term_idle_helper(
     message_iterator: Iterator[ZerverFieldsT],
     user_from_message: Callable[[ZerverFieldsT], Optional[ExternalId]],
+    timestamp_from_message: Callable[[ZerverFieldsT], float],
     zulip_user_id_from_user: Callable[[ExternalId], int],
     users: List[ZerverFieldsT],
     zerver_userprofile: List[ZerverFieldsT],
@@ -767,7 +768,7 @@ def long_term_idle_helper(
     recent_senders: Set[ExternalId] = set()
     NOW = float(timezone_now().timestamp())
     for message in message_iterator:
-        timestamp = float(message["ts"])
+        timestamp = timestamp_from_message(message)
         user = user_from_message(message)
         if user is None:
             continue

zerver/data_import/slack.py

@@ -648,6 +648,7 @@ def process_long_term_idle_users(
     return long_term_idle_helper(
         get_messages_iterator(slack_data_dir, added_channels, added_mpims, dm_members),
         get_message_sending_user,
+        get_timestamp_from_message,
         lambda id: slack_user_id_to_zulip_user_id[id],
         users,
         zerver_userprofile,
@@ -1141,6 +1142,10 @@ def get_message_sending_user(message: ZerverFieldsT) -> Optional[str]:
     return None
 
 
+def get_timestamp_from_message(message: ZerverFieldsT) -> float:
+    return float(message["ts"])
+
+
 def fetch_shared_channel_users(
     user_list: List[ZerverFieldsT], slack_data_dir: str, token: str
 ) -> None: