Added ability for system message username to be localized (#5239)

webapp/i18n/en.json

@@ -1701,6 +1701,7 @@
   "post_info.mobile.unflag": "Unflag",
   "post_info.permalink": "Permalink",
   "post_info.reply": "Reply",
+  "post_info.system": "System",
   "post_message_view.edited": "(edited)",
   "posts_view.loadMore": "Load more messages",
   "posts_view.newMsg": "New Messages",