Mark issuable-resource-links as unique on create

What does this MR do and why?

Mark issuable-resource-links as unique on create

Context: #437902

Depends on: !141549 (merged) in release 16.9

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.

As this work needs to be in a subsequent release from the dependency, we aim to release this in no sooner than release 16.10.

EE: true

Changelog: fixed