allow css overrides for k12
allow css overrides for k12

fixes CNVS-18212

test plan
 - enable k12 feature flag
 - account should show allow css uploads

Change-Id: Icb03c75bda77410e33dce765464560997ed12397
Reviewed-on: https://gerrit.instructure.com/47903
Tested-by: Jenkins
Reviewed-by: Cody Cutrer <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>
Product-Review: Colleen Palmer <387ba1943871e60965e6b44638a2f2e1ab9b8469@instructure.com>
QA-Review: August Thornton <9adc7a1161ddf32ff608de792a7e50179545f026@instructure.com>

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