create feature flag for people page upgrade
create feature flag for people page upgrade

fixes VICE-2996
flag=react_people_page

Test Plan:
- Navigate to Site Admin -> Settings -> Feature Options
- Search for 'People Page Upgrade'
- Feature flag name and description should be present
- Feature flag can be toggled on and off

Change-Id: I6245c68e23e08f5dff71cf3576719cb1ac55ef29
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/296110
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
Reviewed-by: Drake Harper <798ef92bc86a79e8ff768a68dc0a5fcec1d51660@instructure.com>
Product-Review: Drake Harper <798ef92bc86a79e8ff768a68dc0a5fcec1d51660@instructure.com>
QA-Review: Caleb Guanzon <54fa1f30d451e4f3c341a8f3c47c9215fd0be9c6@instructure.com>

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