Reduce exhaustiveness of param validity cucumber

Now that we have exhaustive unit tests for validation of input we can
have a less verbose and more descriptive scenario in the cukes.
Add filters for other methods

All `Application` methods should validate their input so we can 422 on
bad parameters so clients don't pointlessly retry.

Also change the wording of the error message given by `ValidInputFilter`
because it's now used for more actions other than creating subscriber
lists.
Remove ProcessableInputFilter

It's not used any more, having been superseded by `ValidInputFilter`.