Use .length instead of .count for consistency in partial

Now that we're calling to_a on the hits in the controller, this count
does work, but it's a bit brittle to leave it in because a bare count
on a query which selects specific fields instead of * is turned into
invalid SQL by ActiveRecord. This is newly documented:

   https://github.com/rails/rails/commit/3023363fff502487740bd0b83a3069320551ec96

When we're selecting only some fields, .count generates COUNT(field1, field2)
which is invalid SQL. The recommended solution for this is to use .count(:all)
explicitly. In this case the rest of the partial uses .length, so I'm using
that instead here for consistency.