Rake task to change organisation slugs

Following on from https://github.com/alphagov/whitehall/pull/1831 this PR adds a rake task to Whitehall to change an organisation slug. 

Adding commands in this way allows the code to be merged and deployed without needing to coordinate/synchronise the deployments. We can then invoke the commands on the appropriate apps when we're ready to carry out the change.

I'll be adding scripts to the various apps which are involved in an org slug change as per https://github.com/alphagov/wiki/wiki/Changing-GOV.UK-URLs#changing-an-organisations-slug


`#find_each` makes sense

I can't see an `#add_batch` method anywhere (apart from on `FakeRummageableIndex`). Is that still used now that all the search indexing runs in background jobs?