Allow enrollments to be recreated as active

Fixes CNVS-13015

Test plan:
- Add a user as 'invited' to a course via API.
- Delete the enrollment, and recreate it as 'active'
- Verify the enrollment is recreated as 'active'

Change-Id: Ifb4345abf2f586f5b50cfb9a36b7c2bf00f123a4
Reviewed-on: https://gerrit.instructure.com/40255
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: Cosme Salazar <cosme@instructure.com>