Always obey rating locks; make rerating locked posts an error.

Currently rating locks are only obeyed when using the rating: metatag.
They aren't obeyed when:

* Changing the rating via the API.
* Changing the rating via 'Rate Safe' in the mode menu (uses the API).
* Reverting to previous versions.

Also, the current behavior is to ignore the rating: metatag if the post
is locked. This patch instead makes the update fail completely (note that
this could affect trying to mass revert posts that may be rating locked).

Note: the check for `!is_rating_locked_changed?` is so that

  PUT /posts/1.json?post[rating]=s&post[is_rating_locked]=true

works (ie., locking and changing the rating at the same time is okay).

app/models/post.rb

@@ -47,6 +47,7 @@ class Post < ActiveRecord::Base
   validates_uniqueness_of :md5
   validates_inclusion_of :rating, in: %w(s q e), message: "rating must be s, q, or e"
   validate :post_is_not_its_own_parent
+  validate :updater_can_change_rating
   attr_accessible :source, :rating, :tag_string, :old_tag_string, :old_parent_id, :old_source, :old_rating, :parent_id, :has_embedded_notes, :as => [:member, :builder, :gold, :platinum, :janitor, :moderator, :admin, :default]
   attr_accessible :is_rating_locked, :is_note_locked, :as => [:builder, :janitor, :moderator, :admin]
   attr_accessible :is_status_locked, :as => [:admin]
@@ -769,9 +770,7 @@ class Post < ActiveRecord::Base
           self.source = $1
 
         when /^rating:([qse])/i
-          unless is_rating_locked?
-            self.rating = $1.downcase
-          end
+          self.rating = $1.downcase
         end
       end
     end
@@ -1762,6 +1761,15 @@ class Post < ActiveRecord::Base
     save
   end
 
+  def updater_can_change_rating
+    if rating_changed? && is_rating_locked?
+      # Don't forbid changes if the rating lock was just now set in the same update.
+      if !is_rating_locked_changed?
+        errors.add(:rating, "is locked and cannot be changed. Unlock the post first.")
+      end
+    end
+  end
+
   def update_column(name, value)
     ret = super(name, value)
     notify_pubsub