set up grading_period model, migration, and tests

closes CNVS-15906

test plan:
-run rake db:migrate
-verify the migration successfully runs, creating the grading_periods table and adding the index for course_id and account_id

Change-Id: If991a00fc79bc4aecabe8189a43432e2158709c8
Reviewed-on: https://gerrit.instructure.com/42161
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jacob Fugal <jacob@instructure.com>
Reviewed-by: Josh Simpson <jsimpson@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Spencer Olson <solson@instructure.com>