create the plagiarism platform feature flag
create the plagiarism platform feature flag

fixes PLAT-1900

test plan:
The feature flag is only avalible to site admins, on beta/dev/test

Change-Id: I407e1b98349377703d037c7b9443efc0b787e745
Reviewed-on: https://gerrit.instructure.com/93258
Tested-by: Jenkins
Reviewed-by: Weston Dransfield <e9203059795dc94e28a8831c65eb7869e818825e@instructure.com>
QA-Review: August Thornton <9adc7a1161ddf32ff608de792a7e50179545f026@instructure.com>
Product-Review: Nathan Mills <10b0771d20c18ad0d98c1d000665e443052b40b0@instructure.com>

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