Add filter to vote admin that uses a select box for display.

Admin filters [have a template property](https://github.com/django/django/blob/1.4.5/django/contrib/admin/filters.py#L21) that the [admin change list template](https://github.com/django/django/blob/1.4.5/django/contrib/admin/templates/admin/change_list.html#L81) uses to [lookup and render](https://github.com/django/django/blob/1.4.5/django/contrib/admin/templatetags/admin_list.py#L365) the list of filter options. [The default filter template](https://github.com/django/django/blob/1.4.5/django/contrib/admin/templates/admin/filter.html) uses an unordered list of links to display the available filter options, but a select box is better for things like related fields that may have a TON of choices.

This PR adds a filter that uses a select box and is subsequently used to filter votes by the video they are assigned to in the admin interface.