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.
Merge pull request #182 from alphagov/register-specialist-sectors-in-rummager

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