Document filter message for no results

www.agileplannerapp.com/boards/173808/cards/8672
addresses: https://govuk.zendesk.com/tickets/869065

In its current version the document filter shows
a loader image when the search is being performed
and doesn't provide any feedback if results are
not found.

Editors can get confused if they're searching for
a publication to link to a Statistics Announcement
and no results are returned. It may seem as if the
filter is not functioning.

I've added a suitable message which reminds editors
that the lookup is scoped by publication type which
is same as the type of the statistics announcement,
and, publication state being not published.
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
Merge pull request #1847 from alphagov/add-no-results-message-to-document-filter

Document filter message for no results