add grade period support to grade calculator

closes CNVS-17105

Test plan:
  (This has to be done at the rails console)

  * enable multiple grading periods
  * set up assignments in all the grading periods
  * grade the assignments for a student
  * run the grade calculator for a grade period.  make sure the grade
    only considers assignments in the grading period:

    gc = GradeCalculator.new([student_id], course, grading_period)
    pp gc.compute_scores.first.first

Change-Id: I3b87eb7ec08fedebaab4f7a2fc83e68d4d4d5c7e
Reviewed-on: https://gerrit.instructure.com/45753
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Reviewed-by: Cameron Sutter <csutter@instructure.com>
Product-Review: Cameron Matheson <cameron@instructure.com>