Grading Period Grade join table - MGP

fixes CNVS-16454

test plan:
 * test in the console
 - `GradingPeriodGrade.all` should return `[]` (no error)
 - `Enrollment.find(1).grading_period_grades` should return `[]` (no error)
 - make a GradingPeriod:
   `g = GradingPeriod.create(weight: 100, start_date: Time.now, end_date: 1.month.from_now)`
   `g.save!`
 - `g.grading_period_grades` should return `[]` (no error)

Change-Id: I7183d715dbfac3fcd89ab7e453237ad4ce6c650d
Reviewed-on: https://gerrit.instructure.com/45312
Product-Review: Cameron Sutter <csutter@instructure.com>
Reviewed-by: Nick Cloward <ncloward@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>