Add organisation slug changer command
Add rake task and usage notes.
Register editions with panopticon as well as search

Panopticon stores the organisations associated with editions, so they
need to be updated in panopticon. We also changed to use the service
listeners to perform the indexing, as these have well defined behaviours
and are designed to be self-contained.
Use #find_each to iterate editions

there may be a large number of editions
Merge pull request #1840 from alphagov/add-command-to-change-org-slugs

Rake task to change organisation slugs