Rake task to change org slug

Rake task to change an organisation slug.

It is one part of an inter-related set of steps which must be carefully
coordinated. The plan is to make a fabric script to coordinate this process. 

Related story https://www.pivotaltracker.com/story/show/80284724

It performs the following steps:
- changes the organisation slug
- changes the organisation_slug field of any users

we should use the `Need#save_as` method here:

```ruby
n.save_as(:name => "Data Migration",
          :email => "govuk-maslow@digital.cabinet-office.gov.uk",
          :uid => nil)
```
you can see a precedent for this [here](https://github.com/alphagov/govuk_need_api/blob/9ba65b3595bfaa51d065c6e99b104ec0f56b3734/db/migrate/20131202141500_transform_fields_from_monthly_to_yearly.rb#L36).
 so that the need revision is recorded correctly.

That way, the change is less cryptic in the need change notes.