added grade curving warning
added grade curving warning

closes CNVS-6190

test plan:
1. choose to curve grades on an assignment
   - there should be a big scary warning

Change-Id: Ic78efc09a3d535ac760c50482957160e591207e4
Reviewed-on: https://gerrit.instructure.com/21421
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
Reviewed-by: Jon Jensen <44f878afe53efc66b76772bd845eb65944ed8232@instructure.com>
QA-Review: Myller de Araujo <e5c7c5e1b7f2ce5e9870cc98ec52b20b3006cb48@instructure.com>
Product-Review: Ryan Florence <2d8900489f91ac272fd9ce9d5789db6522668ca2@instructure.com>

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