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.

test/unit/post_test.rb

@@ -34,6 +34,15 @@ class PostTest < ActiveSupport::TestCase
         perform_enqueued_jobs # perform IqdbAddPostJob
       end
 
+      should "log a modaction" do
+        @post.expunge!(@user)
+
+        assert_equal(1, ModAction.count)
+        assert_equal("post_permanent_delete", ModAction.last.category)
+        assert_equal(@user, ModAction.last.creator)
+        assert_nil(ModAction.last.subject)
+      end
+
       should "delete the files" do
         assert_nothing_raised { @post.file(:preview) }
         assert_nothing_raised { @post.file(:original) }
@@ -64,6 +73,20 @@ class PostTest < ActiveSupport::TestCase
         assert_equal(0, FavoriteGroup.for_post(@post.id).count)
       end
 
+      should "destroy all modactions belonging to the post" do
+        create(:mod_action, description: "deleted post ##{@post.id}", category: :post_delete, subject: @post)
+        create(:mod_action, description: "undeleted post ##{@post.id}", category: :post_undelete, subject: @post)
+        create(:mod_action, description: "banned post ##{@post.id}", category: :post_ban, subject: @post)
+        create(:mod_action, description: "unbanned post ##{@post.id}", category: :post_unban, subject: @post)
+
+        @post.expunge!(@user)
+
+        assert_equal(1, ModAction.count)
+        assert_equal("post_permanent_delete", ModAction.last.category)
+        assert_equal(@user, ModAction.last.creator)
+        assert_nil(ModAction.last.subject)
+      end
+
       should "decrement the uploader's upload count" do
         assert_difference("@post.uploader.reload.post_upload_count", -1) do
           @post.expunge!