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

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added Community contribution workflowin dev labels

assigned to @tomasz.skorupa

added linked-issue label

	2 Warnings
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	1 Message
	This merge request adds or changes files that require a review from the Database team.

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added 422 commits

• 9fa669f8...0e7931c6 - 422 commits from branch gitlab-org:master

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added 1 commit

• cc2cec81 - Mark issuable-resource-links as unique on create

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changed the description

@gitlab-bot ready

added workflowready for review label and removed workflowin dev label

Hi Coach @terrichu, this Community contribution is ready for review or needs your coaching.

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requested review from @terrichu

I think this also need a database review so adding the appropriate labels. Edit: I've found out that models do not necessarily require a review, but because this is part of a larger database migration plan, I'd like to have it looked at

added database databasereview pending labels

changed milestone to %16.9

added grouprespond label

added devopsservice management sectionseg labels

@carlad-gl would you mind taking the initial backend review?

requested review from @carlad-gl

requested review from @jdrpereira and removed review request for @terrichu