Mark issuable-resource-links as unique on create

Context: https://gitlab.com/gitlab-org/gitlab/-/issues/437902\#in-release-n1

Depends on: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/141549

We mark every newly created issuable-resource-link as is_unique=true.
There is a partial unique index on the database table that will ensure that
_going forward_ no two records are duplicates.

EE: true
Changelog: fixed