Remove "Panopticon" from the 'formats not to index' list

In order to index specialist sector artefacts in the search index, we 
need to remove Panopticon from the list of formats not to index.

At present, the only format owned by Panopticon is specialist sectors, 
so this will not affect any other formats.
Bump govuk_content_models to 13.3.0

This includes the new ‘state’ field on the Tag model.
Add data migration to set all tags to live
Merge pull request #183 from alphagov/add-state-attribute-to-tags

Add state attribute to tags
Merge pull request #182 from alphagov/register-specialist-sectors-in-rummager

Remove "Panopticon" from the 'formats not to index' list