Organisations from Whitehall

This is the first part of managing user membership of organisations in Signon, which will enable other apps (e.g. Transition) to implement access controls based on a user's organisations.

What this does:

* enables fetching and updating of organisations from Whitehall's [organisations API](https://github.com/alphagov/whitehall/blob/master/app/controllers/api/organisations_controller.rb) using an [API adapter](https://github.com/alphagov/gds-api-adapters/blob/master/lib/gds_api/organisations.rb)
* provides an index view of organisations for admins
* provides UI for editing a user's membership of organisations (currently hidden by a feature flag)

There are some further stages of this process which are not yet completed and are dependent on this:

* adding a cron job in puppet to regularly run the rake task added here to fetch organisations data and update them
* exposing the user's organisations in the user JSON returned by Signon after authentication
    * adding support for this change to [gds-sso](https://github.com/alphagov/gds-sso) and [omniauth-gds](https://github.com/alphagov/omniauth-gds)
    * checking that this change won't break the apps which use Signon without gds-sso
* making Signon the canonical place to edit user memberships of organisations
    * importing existing membership data from Whitehall
    * enabling editing of a user's organisations membership in Signon (currently hidden by a feature flag)
    * disabling editing of this in Whitehall