PERF: Try to match users before groups.

User mentions are more common than group mentions so
this will allow us to avoid an extra query.

lib/pretty_text/helpers.rb

@@ -41,8 +41,8 @@ module PrettyText
 
     def mention_lookup(name)
       return false   if name.blank?
-      return "group" if Group.exists?(name: name)
       return "user"  if User.exists?(username_lower: name.downcase)
+      return "group" if Group.exists?(name: name)
     end
 
     def category_hashtag_lookup(category_slug)