Add "not equals" option to ActiveAdmin string filter

active_admin.en.yml

en:
  active_admin:
    filters:
      predicates:
        not_eq: "Not equals"

filter_string_with_negative_input.rb

module ActiveAdmin
  module Inputs
    class FilterStringWithNegativeInput < FilterStringInput
      filter :contains, :equals, :starts_with, :ends_with, :not_eq
    end
  end
end

Just for everyone looking second file is not required! Just add the translation to your en.yml and then in each field you need this add something like this:
filter :first_name, filters: [:equals, :contains, :not_eq]

Thanks for this! Very helpful.