posts: mark media asset as expunged when post is expunged.

Fix it so that when a post is expunged, the media asset is also marked
as expunged. This way the files will be deleted, but the media asset
will still remain as a record of what was expunged. The media asset will
have the md5, width, height, file ext, and file size of the deleted file.

test/unit/post_test.rb

@@ -47,6 +47,12 @@ class PostTest < ActiveSupport::TestCase
         assert_raise(StandardError) { @post.file(:original) }
       end
 
+      should "mark the media asset as expunged" do
+        @post.expunge!
+
+        assert_equal("expunged", @post.media_asset.status)
+      end
+
       should "remove all favorites" do
         @post.expunge!