performance: Avoid computing page_params.streams in webapp.

The query to get "occupied" streams has been expensive
in the past.  I'm not sure how much any recent attempts
to optimize that query have mitigated the issue, but
since we clearly aren't sending this data, there is no
reason to compute it.

zerver/lib/events.py

@@ -89,7 +89,8 @@ def fetch_initial_state_data(
     user_avatar_url_field_optional: bool,
     realm: Realm,
     slim_presence: bool = False,
-    include_subscribers: bool = True
+    include_subscribers: bool = True,
+    include_streams: bool = True,
 ) -> Dict[str, Any]:
     """When `event_types` is None, fetches the core data powering the
     webapp's `page_params` and `/api/v1/register` (for mobile/terminal
@@ -354,10 +355,14 @@ def fetch_initial_state_data(
         state['starred_messages'] = [] if user_profile is None else get_starred_message_ids(user_profile)
 
     if want('stream'):
-        if user_profile is not None:
-            state['streams'] = do_get_streams(user_profile)
-        else:
-            state['streams'] = get_web_public_streams(realm)
+        if include_streams:
+            # The webapp doesn't use the data from here; instead,
+            # it uses data from state["subscriptions"] and other
+            # places.
+            if user_profile is not None:
+                state['streams'] = do_get_streams(user_profile)
+            else:
+                state['streams'] = get_web_public_streams(realm)
         state['stream_name_max_length'] = Stream.MAX_NAME_LENGTH
         state['stream_description_max_length'] = Stream.MAX_DESCRIPTION_LENGTH
     if want('default_streams'):
@@ -914,6 +919,7 @@ def do_events_register(
     queue_lifespan_secs: int = 0,
     all_public_streams: bool = False,
     include_subscribers: bool = True,
+    include_streams: bool = True,
     client_capabilities: Dict[str, bool] = {},
     narrow: Iterable[Sequence[str]] = [],
     fetch_event_types: Optional[Iterable[str]] = None
@@ -961,7 +967,8 @@ def do_events_register(
         user_avatar_url_field_optional=user_avatar_url_field_optional,
         realm=user_profile.realm,
         slim_presence=slim_presence,
-        include_subscribers=include_subscribers
+        include_subscribers=include_subscribers,
+        include_streams=include_streams,
     )
 
     # Apply events that came in while we were fetching initial data