add grading period group model

add GradingPeriodGroup, and change associations between GradingPeriods,
GradingPeriodGroups, Courses, and Accounts. also adjust the grading
periods controller to account for addition of grading period groups

closes CNVS-16538

test plan:
-run bundle exec rake db:migrate, and bundle exec rake db:migrate RAILS_ENV=test
-verify the migrations successfully run
-open the rails console in sandbox: bundle exec rails c -s
-create a course, a few grading periods, and a grading period group. Add the grading periods to the group. Assign
  the grading period group to the course.
  $ course = Course.create
  $ grading_period1 = GradingPeriod.create(weight: 25.0, start_date: Time.zone.now, end_date: 2.days.from_now)
  $ grading_period2 = GradingPeriod.create(weight: 30.0, start_date: Time.zone.now, end_date: 2.days.from_now)
  $ grading_period_group = GradingPeriodGroup.create()
  $ grading_period_group.grading_periods << grading_period1
  $ grading_period_group.grading_periods << grading_period2
  $ grading_period_group.course = course
-verify the associations are working as expected, i.e. a GradingPeriodGroup has GradingPeriods, a GradingPeriod
  belongs to a GradingPeriodGroup, and a GradingPeriodGroup belongs to a course or account.
  $ grading_period_group.grading_periods #should return an array containing grading_period1 and grading_period2
  $ grading_period1.grading_period_group #should return grading_period_group
  $ grading_period2.grading_period_group #should return grading_period_group
  $ grading_period_group.course #should return course
  $ grading_period_group.account #should return nil (should not throw error)

Change-Id: I9d7465431dabd2afa18e7a8a33706b9a78a94cd1
Reviewed-on: https://gerrit.instructure.com/43512
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Josh Simpson <jsimpson@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Reviewed-by: Jacob Fugal <jacob@instructure.com>
Product-Review: Spencer Olson <solson@instructure.com>