mod actions: record the subject of the mod action.

Add a polymorphic `subject` field that records the subject of the mod
action. The subject is the post, user, comment, artist, etc the mod
action is for.

* The subject for the user ban and unban actions is the user, not the ban itself.
* The subject for the user feedback update and deletion actions is the user,
  not the feedback itself.
* The subject for the post undeletion action is the post, not the approval itself.
* The subject for the move favorites action is the source post where the
  favorites were moved from, not the destination post where the favorites
  were moved to.
* The subject for the post permanent delete action is nil, because the
  post itself is hard deleted.
* When a post is permanently deleted, all mod actions related to the
  post are deleted as well.

app/controllers/moderator/post/posts_controller.rb

@@ -19,12 +19,12 @@ module Moderator
 
       def expunge
         @post = authorize ::Post.find(params[:id])
-        @post.expunge!
+        @post.expunge!(CurrentUser.user)
       end
 
       def ban
         @post = authorize ::Post.find(params[:id])
-        @post.ban!
+        @post.ban!(CurrentUser.user)
         flash[:notice] = "Post was banned"
 
         respond_with(@post)
@@ -32,7 +32,7 @@ module Moderator
 
       def unban
         @post = authorize ::Post.find(params[:id])
-        @post.unban!
+        @post.unban!(CurrentUser.user)
         flash[:notice] = "Post was unbanned"
 
         respond_with(@post)