Create anonymous discussion feature flag
Create anonymous discussion feature flag

Closes VICE-2249
flag=none

Test Plan:
 - able to enable anonymous discussion ff

Change-Id: I4397865ebd9c80e17c88d3aff9a5d1afaa284f31
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/278468
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
Reviewed-by: Jeffrey Johnson <7a0f98bfe168bc29d5611ba0275e88567b492abc@instructure.com>
QA-Review: Jeffrey Johnson <7a0f98bfe168bc29d5611ba0275e88567b492abc@instructure.com>
Product-Review: Jeffrey Johnson <7a0f98bfe168bc29d5611ba0275e88567b492abc@instructure.com>

instructure/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms