add multiple grading periods feature flag

closes CNVS-15959
Add a feature flag (development / beta only) to use with the multiple
grading periods feature.

Test Plan
- In a development or beta environment (to include QA portals), you
  should see the feature flag for 'multiple grading periods'.  It should
  default to 'Allowed', and be tagged as 'development'.

Change-Id: I559e278c37d297c68e6c5e4262969839a038bafa
Reviewed-on: https://gerrit.instructure.com/42122
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cameron Matheson <cameron@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Josh Simpson <jsimpson@instructure.com>