Fix subreddit names in ModAction json output

The sr_id36 field is working for viewing a json version of the
mod log, but the subreddit name field was null. This adds the
subreddit name back in.

r2/r2/lib/jsontemplates.py

@@ -1460,7 +1460,7 @@ class ModActionTemplate(ThingJsonTemplate):
         mod='moderator',
         mod_id36='mod_id36',
         sr_id36='sr_id36',
-        subreddit='sr_name',
+        subreddit='subreddit',
         target_author='target_author',
         target_fullname='target_fullname',
         target_permalink='target_permalink',
@@ -1483,6 +1483,8 @@ class ModActionTemplate(ThingJsonTemplate):
                 return None
         elif attr == "moderator":
             return thing.moderator.name
+        elif attr == "subreddit":
+            return thing.subreddit.name
 
         return ThingJsonTemplate.thing_attr(self, thing, attr)