Add organisation slug changer rake task

This rake task performs the following steps:

- updates the org slug
- updates associated artefacts to use the new slug (since org slug is used as
  foreign key)
- reindexes the updated artefacts in search

It is one part of an inter-related set of steps which must be carefully coordinated. The idea of using rake tasks in each app is that all of the rake tasks can then be deployed independently and then the execution of the rake tasks can be coordinated without needing to tie-in with deployments.

For reference:

https://github.com/alphagov/wiki/wiki/Changing-GOV.UK-URLs#changing-an-organisations-slug"