Don't register with panopticon when changing slug

Artefacts in panopticon will be updated by a rake task run directly in
panopticon, therefore we don't need to re-register content with
panopticon. The direct rake task will be much more efficient.
Re-registering with panopticon is a potentially costly operation.
Merge pull request #1850 from alphagov/dont-register-with-panopticon-when-changing-slug

Don't register with panopticon when changing slug