add ff for multiple assignment deep linking
add ff for multiple assignment deep linking

refs INTEROP-6831
flag=none

TEST PLAN:
- No functionality yet

Change-Id: Ibda7e0b3e5ca90c98d6a2df8f3e72216c35ba4dc
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/270918
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
QA-Review: Xander Moffatt <27f70fe194da423604e6e0a63a3de19814736fa9@instructure.com>
Product-Review: Alexandre Trindade <f185c528be8ad1468cdf5c6cccdc72d6de8c2461@instructure.com>
Reviewed-by: Xander Moffatt <27f70fe194da423604e6e0a63a3de19814736fa9@instructure.com>

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