Bugfix: Use the 'mounted on' column for checking uniqueness.

When using an uploader name which does not map onto an existing column and using the `:mount_on` option for mapping to an existing one the uniqueness validator fails with an error because he's using the uploader's name for AR's `where`.

I created a new `Dance` class for this bugfix because re-using `Party` and mounting another uploader intefered too much with a lot of other test cases.