Sorting already exists for search results from the web. Most of this
code for searching is shared. Unfortunately in the web search results we
had to implement this by using a single parameter. In order for our API
to be in line with other APIs it seemed preferable to implement sorting
as 2 params (field, direction).

To make matters worse the web sorting does things like `created_asc` for
sorting by `created_at asc`. So it wasn't even perfectly constructed by
concatenating field name and direction. Unfortunately the simplest
option here seemed to be to push the understanding of how to interpret
these different params into the searching code so it needs to handle 2
different meanings for the `sort` param.

I considered changing the web code to do a translation of params before
passing down to the `SearchService` but this was tricky due `params`
being passed in and also `@sort` is used by the view. We may in future
be able to refactor this to use 2 params but it seemed tricky to
introduce now with a single form field.