min should match always on for /search

Removes the need for frontend to specify it.

This will mean:
  * service-manual will get the benefit of min should match (fewer, more
    relevant results)
  * search Content API will get the benefit without end-users needing to
    specifically request it
  * frontend doesn't need to specify it wants the feature

Arbitrary min should match rules were supported, but not used. I felt it wasn't
worth keeping if it was unused, especially as it wasn't really documented and
it was fairly complex.
Merge pull request #141 from alphagov/min_should_match_all_the_time

min should match always on for /search