Ruby 2.6 introduced `Enumerable#filter`, which takes no arguments.
Attempting to call `filter` on an `ActiveRecord::Relation` with a scope
will fail with a `wrong number of arguments (given 1, expected 0)`
message because the `Enumerable#filter` implementation overrides the
delegated `ActiveRecord::Relation#filter` method.

To make Admin::UsersController compatible with Ruby 2.6, rename
`User.filter` to `User.filter_items`.