Split whips from ministerial roles

Admin interface for selecting a whip org.

We then append whip positions grouped by whip_organisations sorted by
seniority.

ActiveRecordLikeInterface now includes ActiveModel::Conversion and
implements model_name so that content_tag_for method works.

https://www.pivotaltracker.com/story/show/44368101

This also fixes a bug as we have split the `ministerial_roles` association with `ministerial_whip_roles` on organisations.
https://www.pivotaltracker.com/story/show/44699729