DEV: Use AR enums in reviewables related code

This is the first patch of many to replace our custom enums in Ruby by
the ones provided by `ActiveRecord`.

app/services/spam_rule/flag_sockpuppets.rb

@@ -47,8 +47,7 @@ class SpamRule::FlagSockpuppets
   private
 
   def flag_post(post, message)
-    can_trust_user = ReviewableFlaggedPost.where(status: Reviewable.statuses[:rejected], target_created_by: post.user).exists?
-    return if can_trust_user
+    return if ReviewableFlaggedPost.rejected.exists?(target_created_by: post.user)
 
     PostActionCreator.create(Discourse.system_user, post, :spam, message: message)
   end