backend: Change return value of compose_views.

As a preparatory step to refactoring json_success to accept
request as a parameter, update helper function `compose_views`
in `views.streams.py` to return the response data and call
json_success from view functions that utilize `compose_views`.

Also, updates related test in `zerver.tests.test_subs.py`.

zerver/views/streams.py

@@ -374,10 +374,12 @@ def update_subscriptions_backend(
         lambda: add_subscriptions_backend(request, user_profile, streams_raw=add),
         lambda: remove_subscriptions_backend(request, user_profile, streams_raw=delete),
     ]
-    return compose_views(thunks)
+    data = compose_views(thunks)
+
+    return json_success(data)
 
 
-def compose_views(thunks: List[Callable[[], HttpResponse]]) -> HttpResponse:
+def compose_views(thunks: List[Callable[[], HttpResponse]]) -> Dict[str, Any]:
     """
     This takes a series of thunks and calls them in sequence, and it
     smushes all the json results into a single response when
@@ -391,7 +393,7 @@ def compose_views(thunks: List[Callable[[], HttpResponse]]) -> HttpResponse:
         for thunk in thunks:
             response = thunk()
             json_dict.update(orjson.loads(response.content))
-    return json_success(json_dict)
+    return json_dict
 
 
 check_principals: Validator[Union[List[str], List[int]]] = check_union(

zerver/views/user_groups.py

@@ -103,7 +103,9 @@ def update_user_group_backend(
             request, user_profile, user_group_id=user_group_id, members=delete
         ),
     ]
-    return compose_views(thunks)
+    data = compose_views(thunks)
+
+    return json_success(data)
 
 
 def add_members_to_group_backend(