Split whips from minterial 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
implments model_name so that content_tag_for method works.
Tidy up code
remove unused methods
Use group_by for collecting the whip orgs

Note: this change means that whip orgs without any roles will not be
displayed.
Refactor MinisterialRolesController and add cucumber tests

The refactoring makes the behaviour clearer.

Added better test coverage in the cucumber tests as there were no tests
covering the index pages.
Improve layout of whips on ministers page
Merge pull request #239 from alphagov/split-whips-on-ministers-page

Split whips from ministerial roles

https://www.pivotaltracker.com/story/show/44368101
https://www.pivotaltracker.com/story/show/44699729