UX: suppress "in reply to" section in emails by default

Previously we would include this section, unfortunately

1. It is usually elided in gmail
2. It can make the emails longer and more confusing
3. Omission is a feature, it means people need to visit site to get context

config/site_settings.yml

@@ -2063,7 +2063,7 @@ user_preferences:
     enum: "PreviousRepliesSiteSetting"
     default: 2
   default_email_in_reply_to:
-    default: true
+    default: false
 
   default_other_new_topic_duration_minutes:
     enum: "NewTopicDurationSiteSetting"