Use ActsAsList#update_order in AssignmentGroups#reorder

refs CNVS-8955

Test plan:
  * Try to reorder assignment groups in the assignment index page (old
    one and draft state).
  * Reload the page.
  * The groups should be arranged in your new order.

Change-Id: I61f637202e954575b500411ea2f7da4d962c4cdf
Reviewed-on: https://gerrit.instructure.com/25984
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cameron Sutter <csutter@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Product-Review: Cameron Matheson <cameron@instructure.com>

app/controllers/assignment_groups_controller.rb

@@ -118,17 +118,10 @@ class AssignmentGroupsController < ApplicationController
   def reorder
     if authorized_action(@context.assignment_groups.new, @current_user, :update)
       order = params[:order].split(',')
-      ids = []
-      order.each_index do |idx|
-        id = order[idx]
-        group = @context.assignment_groups.active.find_by_id(id) if id.present?
-        if group
-          group.move_to_bottom
-          ids << group.id
-        end
-      end
+      @context.assignment_groups.first.update_order(order)
       respond_to do |format|
-        format.json { render :json => {:reorder => true, :order => ids}, :status => :ok }
+        new_order = @context.assignment_groups.pluck(:id)
+        format.json { render :json => {:reorder => true, :order => new_order}, :status => :ok }
       end
     end
   end

spec/controllers/assignment_groups_controller_spec.rb

@@ -70,16 +70,13 @@ describe AssignmentGroupsController do
 
     it "should reorder assignment groups" do
       course_with_teacher_logged_in(:active_all => true)
-      g1 = course_group
-      g2 = course_group
-      g1.position.should eql(1)
-      g2.position.should eql(2)
+      groups = 3.times.map { course_group }
+      groups.map(&:position).should == [1, 2, 3]
+      g1, g2, _ = groups
       post 'reorder', :course_id => @course.id, :order => "#{g2.id},#{g1.id}"
       response.should be_success
-      g1.reload
-      g2.reload
-      g1.position.should eql(2)
-      g2.position.should eql(1)
+      groups.each &:reload
+      groups.map(&:position).should == [2, 1, 3]
     end
 
   end