Send "missing": "_last" with timestamp ordered searches.

This puts documents at the bottom of the results
if they have no `public_timestamp` field rather
than not returning them at all.
Merge pull request #312 from alphagov/content-without-timestamps-on-latest-feed

Send "missing": "_last" with timestamp ordered searches.
Don't filter out documents missing the sort field

This was necessary with an older elasticsearch which (and my memory is
shaky here) either errored or gave unexpected results for documents
missing the sort field.  Since we're now wanting documents missing the
sort field to appear at the end of the list of documents, remove this
filter.
Merge pull request #313 from alphagov/dont-require-sort-fields-to-exist-on-documents

Don't filter out documents missing the sort field