Update UserDatatable for ajax-datatables-rails 1

Resolves failing test `spec/datatables/user_datatable_spec.rb:97`:

    1) UserDatatable outputs recordsTotal
       Failure/Error: filter_records(fetch_records).unscope(:group, :select).count(:all)

       NotImplementedError:
         NotImplementedError
       # ./app/datatables/user_datatable.rb:59:in `records_filtered_count'
       # ./spec/datatables/user_datatable_spec.rb:95:in `block (3 levels) in <top (required)>'
       # ./spec/datatables/user_datatable_spec.rb:98:in `block (3 levels) in <top (required)>'

Details: https://github.com/jbox-web/ajax-datatables-rails/blob/v1.3.1/doc/migrate.md#to-migrate-from-v04x-to-v100

RegistrationDatatable has already been updated in 81853d1ef9.

app/controllers/admin/users_controller.rb

@@ -23,7 +23,7 @@ module Admin
       respond_to do |format|
         format.html
         format.json do
-          render json: UserDatatable.new(view_context: view_context)
+          render json: UserDatatable.new({}, view_context: view_context)
         end
       end
     end

app/datatables/user_datatable.rb

@@ -1,12 +1,17 @@
 # frozen_string_literal: true
 
-class UserDatatable < AjaxDatatablesRails::Base
+class UserDatatable < AjaxDatatablesRails::ActiveRecord
   extend Forwardable
 
   def_delegator :@view, :show_roles
   def_delegator :@view, :admin_user_path
   def_delegator :@view, :edit_admin_user_path
 
+  def initialize(params, opts = {})
+    @view = opts[:view_context]
+    super
+  end
+
   def view_columns
     # Declare strings in this format: ModelName.column_name
     # or in aliased_join_table.column_name format

spec/datatables/user_datatable_spec.rb

@@ -4,7 +4,7 @@ require 'spec_helper'
 
 describe UserDatatable do
   subject! do
-    described_class.new(view)
+    described_class.new({}, view_context: view)
   end
 
   let(:data_cols) do