Add page_params.unread_msgs.count.

This field is convenient for bankruptcy checks.  Clients could
calculate it from page_params.unread_msgs before this change, but
it would kind of a painful calculation.

To add count, we had to simplify the mypy annotations, which weren't
really accurate before.

docs/unread_messages.md

@@ -14,6 +14,7 @@ in the register call.
 
 ```
 {
+    "count": 4,
     "huddles": [
         {
             "user_ids_string": "3,4,6",

zerver/lib/events.py

@@ -187,12 +187,13 @@ def fetch_initial_state_data(user_profile, event_types, queue_id,
 
 
 def remove_message_id_from_unread_mgs(state, remove_id):
-    # type: (Dict[str, Dict[str, List[Any]]], int) -> None
+    # type: (Dict[str, Dict[str, Any]], int) -> None
     for message_type in ['pms', 'streams', 'huddles']:
         threads = state['unread_msgs'][message_type]
         for obj in threads:
             msg_ids = obj['unread_message_ids']
             if remove_id in msg_ids:
+                state['unread_msgs']['count'] -= 1
                 msg_ids.remove(remove_id)
         state['unread_msgs'][message_type] = [
             obj for obj in threads

zerver/lib/message.py

@@ -395,7 +395,7 @@ def get_inactive_recipient_ids(user_profile):
     return inactive_recipient_ids
 
 def get_unread_message_ids_per_recipient(user_profile):
-    # type: (UserProfile) -> Dict[str, List[Dict[str, Any]]]
+    # type: (UserProfile) -> Dict[str, Any]
 
     excluded_recipient_ids = get_inactive_recipient_ids(user_profile)
 
@@ -419,6 +419,7 @@ def get_unread_message_ids_per_recipient(user_profile):
     user_msgs = list(user_msgs[:MAX_UNREAD_MESSAGES])
 
     rows = list(reversed(user_msgs))
+    count = len(rows)
 
     pm_msgs = [
         dict(
@@ -484,12 +485,15 @@ def get_unread_message_ids_per_recipient(user_profile):
         streams=stream_objects,
         huddles=huddle_objects,
         mentions=mentioned_message_ids,
+        count=count,
     )
 
     return result
 
 def apply_unread_message_event(state, message):
-    # type: (Dict[str, List[Dict[str, Any]]], Dict[str, Any]) -> None
+    # type: (Dict[str, Any], Dict[str, Any]) -> None
+    state['count'] += 1
+
     message_id = message['id']
     if message['type'] == 'stream':
         message_type = 'stream'