Add "not equals" option to ActiveAdmin string filter

active_admin.en.yml

# in my experience, adding this will duplicate the filter list next to the input
en:
  active_admin:
    filters:
      predicates:
        not_eq: "Not equals"

filter_string_with_negative_input.rb

# save this in lib/active_admin/inputs/
# to use this in the admin/dish.rb for example
# filter :name, as: :string_with_negative

module ActiveAdmin
  module Inputs
    module Filters
      class StringWithNegativeInput < StringInput
        filter :contains, :equals, :starts_with, :ends_with, :not_eq
      end
    end
  end
end