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


ah, ok I see `Rummageable` defines `#add_batch`, but we are not invoking it directly, rather we're doing it via background job.

I guess this may cause performance problems if the organisation has a large number of documents associated. Possibly we should just have a cut-off threshold and refuse to run if there are more than a certain number of documents (500?). Bearing in mind that we also need to re-register each edition with panopticon which can't be done easily in batch, so it would require quite a bit of engineering to make that work efficiently. 

Ofqual who we are wanting to migrate in the immediate term has 242 documents.