Remove scopes from PanopticonMapping

The `manuals` scope was unused. The `documents` scope was only used by
the `all_document_ids` method. The only code using the
`all_document_ids` method was the `SpecialistDocumentRepository`.

Rather than getting the list of document ids from `PanopticonMapping` we
can use a `distinct` query against `SpecialistDocumentEdition`.
Remove unused dependency from SpecialistDocumentRepository

It no longer uses `panopticon_mappings` now that it gets the list of
document ids from `SpecialistDocumentEdition`.

Also swap the dependencies to using a hash style instead of ordered
arguments.
Merge pull request #161 from alphagov/remove-panopticon-mapping-scopes

Clean up PanopticonMapping and SpecialistDocumentRepository