Mypy: Ensure consistency of QuerySet return types in models.py.

Other functions took the form of returning Sequence[T] when the QuerySet
functionality is unused beyond the function, with T being the objects
filtered for in the function body; this commit follows that practice for the
one remaining python2 comment-annotated function, completing the transition
of models.py to py3.5 function annotations.

A note is also added to another function regarding a need to return a
QuerySet, and ideally a QuerySet[T] in line with the other functions, as and
when QuerySet becomes annotated as a generic.

tools/linter_lib/custom_check.py

@@ -480,7 +480,6 @@ def build_custom_checkers(by_lang):
              'zerver/lib/cache.py',
              'zerver/lib/request.py',
              'zerver/lib/stream_subscription.py',
-             'zerver/models.py',
              'zerver/tornado/descriptors.py',
              'zerver/views/streams.py',
              # thumbor is (currently) python2 only

zerver/models.py

@@ -1036,6 +1036,8 @@ def stream_name_in_use(stream_name: Text, realm_id: int) -> bool:
     ).exists()
 
 def get_active_streams(realm: Optional[Realm]) -> QuerySet:
+    # TODO: Change return type to QuerySet[Stream]
+    # NOTE: Return value is used as a QuerySet, so cannot currently be Sequence[QuerySet]
     """
     Return all streams (including invite-only streams) that have not been deactivated.
     """
@@ -1249,8 +1251,7 @@ def pre_save_message(sender: Any, **kwargs: Any) -> None:
         message = kwargs['instance']
         message.update_calculated_fields()
 
-def get_context_for_message(message):
+def get_context_for_message(message: Message) -> Sequence[Message]:
-    # type: (Message) -> QuerySet[Message]
     # TODO: Change return type to QuerySet[Message]
     return Message.objects.filter(
         recipient_id=message.recipient_id,