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 
thanks Jenny, that's a good thought. 

I've been following the [notes](https://github.com/alphagov/wiki/wiki/Changing-GOV.UK-URLs#changing-an-organisations-slug) which Alex T made and those do do the change in signon as the second step (before transition), although I hadn't been aware of dependencies between the steps. So I'll make sure that the ordering dependency is noted in the fabric script.