Rake task for org slug change

This is one part of an inter-related set of steps which must be carefully coordinated. The plan is to make a fabric script which invokes each rake task in turn.

Changing the org slug in transition also requires a pull request against `transition-config` to change any corresponding sites. The rake task description includes notes on this, which will also be mentioned in the fabric script.

This rake task:

- changes the whitehall_slug of the organisation itself
- changes the organisation_slug of any users which reference that
  organisation

https://www.pivotaltracker.com/story/show/80284724

/cc @jamiecobbett 
We're not using the `FactoryGirl` style in 'ere - we've mixed `FactoryGirl::Syntax::Methods` in [everywhere](https://github.com/alphagov/transition/blob/master/spec/spec_helper.rb#L47). Just a bare `create` is fine.