streams: Extract some code out of do_get_streams in a new function.

This commit extracts the code which queries the required streams
to a new function "get_user_streams". The new functions returns
the list of "Stream" object and not dictionaries and then
do_get_streams function converts it into list of dictionaries.

This change is important because we would use the new function
in further commit where we want list of "Stream" objects and
not list of dictionaries.

zerver/lib/streams.py

@@ -878,17 +878,14 @@ def get_web_public_streams(realm: Realm) -> List[APIStreamDict]:  # nocoverage
     return stream_dicts
 
 
-def do_get_streams(
+def get_streams_for_user(
     user_profile: UserProfile,
     include_public: bool = True,
     include_web_public: bool = False,
     include_subscribed: bool = True,
     include_all_active: bool = False,
-    include_default: bool = False,
     include_owner_subscribed: bool = False,
-) -> List[APIStreamDict]:
-    # This function is only used by API clients now.
-
+) -> List[Stream]:
     if include_all_active and not user_profile.is_realm_admin:
         raise JsonableError(_("User not authorized for this query"))
 
@@ -941,6 +938,29 @@ def do_get_streams(
             # Don't bother going to the database with no valid sources
             return []
 
+    return list(streams)
+
+
+def do_get_streams(
+    user_profile: UserProfile,
+    include_public: bool = True,
+    include_web_public: bool = False,
+    include_subscribed: bool = True,
+    include_all_active: bool = False,
+    include_default: bool = False,
+    include_owner_subscribed: bool = False,
+) -> List[APIStreamDict]:
+    # This function is only used by API clients now.
+
+    streams = get_streams_for_user(
+        user_profile,
+        include_public,
+        include_web_public,
+        include_subscribed,
+        include_all_active,
+        include_owner_subscribed,
+    )
+
     stream_ids = {stream.id for stream in streams}
     recent_traffic = get_streams_traffic(stream_ids, user_profile.realm)