module ActiveAdmin::OutputSafetyHelper

def to_sentence(array, options = {})

https://github.com/rails/rails/blob/9c35bf2a6a27431c6aa283db781c19f61c5155be/actionview/lib/action_view/helpers/output_safety_helper.rb#L43
Copied from Rails 5 to support Rails 4.

ActiveSupport's {Array#to_sentence}[http://api.rubyonrails.org/classes/Array.html#method-i-to_sentence].
joined by the connector word. This is the html_safe-aware version of
Converts the array to a comma-separated sentence where the last element is 
def to_sentence(array, options = {})
  options.assert_valid_keys(:words_connector, :two_words_connector, :last_word_connector, :locale)
  default_connectors = {
    words_connector: ", ",
    two_words_connector: " and ",
    last_word_connector: ", and "
  }
  if defined?(::I18n)
    i18n_connectors = ::I18n.translate(:'support.array', locale: options[:locale], default: {})
    default_connectors.merge!(i18n_connectors)
  end
  options = default_connectors.merge!(options)
  case array.length
  when 0
    "".html_safe
  when 1
    ERB::Util.html_escape(array[0])
  when 2
    safe_join([array[0], array[1]], options[:two_words_connector])
  else
    safe_join([safe_join(array[0...-1], options[:words_connector]), options[:last_word_connector], array[-1]], nil)
  end
end