add multiple grading periods feature flag
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 <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
Reviewed-by: Cameron Matheson <1d572acbfa68c7c6e541c7b840d6b622e5c0dc91@instructure.com>
QA-Review: Amber Taniuchi <ef4bfd1e6d278f6bc7d4a478f02be2cb9581a0a5@instructure.com>
Product-Review: Josh Simpson <bfb4987b134a00f73d881fff5113b1d5fa1876bc@instructure.com>

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