Improve how params from controller are permitted in filter helper (#3129)

Improve how params from controller are permitted in filter helper (#3129)

The `params` variable here was quite overloaded.

It exists via the controller to hold the request params, and was sometimes being
used in this helper as that object, but other times was being used as a local
variable, or to pass to another method, and this was confusing.

This change renames the args for a method away from `params` for more clarity,
and extracts the actual usage of the controller-provided `params` to a
better-named method for clarity.