actions: Deduplicate parts of get_web_public_subs.

This has the side of effect of making new fields we add to Stream be
automatically included, which will help maintain this code as we
upgrade it.

This commit adds is_web_public, history_public_to_subscribers, and
email_notifications fields to the dictionary.

zerver/lib/actions.py

@@ -4708,26 +4708,27 @@ def get_web_public_subs(realm: Realm) -> SubHelperT:
         color_idx = (color_idx + 1) % len(STREAM_ASSIGNMENT_COLORS)
         return color
 
-    subscribed = [
-        {'name': stream.name,
-         'is_muted': False,
-         'invite_only': False,
-         'is_announcement_only': stream.is_announcement_only,
-         'color': get_next_color(),
-         'desktop_notifications': True,
-         'audible_notifications': True,
-         'push_notifications': False,
-         'pin_to_top': False,
-         'stream_id': stream.id,
-         'description': stream.description,
-         'rendered_description': stream.rendered_description,
-         'is_old_stream': is_old_stream(stream.date_created),
-         'first_message_id': stream.first_message_id,
-         'stream_weekly_traffic': get_average_weekly_stream_traffic(stream.id,
-                                                                    stream.date_created,
-                                                                    {}),
-         'email_address': ''}
-        for stream in Stream.objects.filter(realm=realm, is_web_public=True, deactivated=False)]
+    subscribed = []
+    for stream in Stream.objects.filter(realm=realm, is_web_public=True, deactivated=False):
+        stream_dict = stream.to_dict()
+
+        # Add versions of the Subscription fields based on a simulated
+        # new user subscription set.
+        stream_dict['is_muted'] = False
+        stream_dict['color'] = get_next_color()
+        stream_dict['desktop_notifications'] = True
+        stream_dict['audible_notifications'] = True
+        stream_dict['push_notifications'] = True
+        stream_dict['email_notifications'] = True
+        stream_dict['pin_to_top'] = False
+        stream_dict['is_old_stream'] = is_old_stream(stream.date_created)
+        stream_weekly_traffic = get_average_weekly_stream_traffic(stream.id,
+                                                                  stream.date_created,
+                                                                  {})
+        stream_dict['stream_weekly_traffic'] = stream_weekly_traffic
+        stream_dict['email_address'] = ''
+        subscribed.append(stream_dict)
+
     return (subscribed, [], [])
 
 # In general, it's better to avoid using .values() because it makes