Order by last_update when not searching by keyword

In order to match the finder-api behaviour that rummager is taking over,
we want to order results when filtering by the date they were last
updated.

This last_update field, once added to rummager in the schema and
permitted sort fields, and provided by the publishing applications, will
allow this behaviour to be maintained.

See alphagov/rummager#294 and alphagov/specialist-publisher#250
I mean specifically this line change. My understanding of the previous line and the new line is that they do the same thing.

Let's talk about this when we're both back at our desks.