Fix wiki comments user mentions

What does this MR do and why?

This fixes:

• User mentions for wiki page comments erroring out due to the user mention records not including the sharding key namespace_id.

  Mentions are UPSERTed directly in the db and the model validations are skipped, so instead of a model validation, this is now done with a database trigger.

References

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• Wiki page comments unable to mention another us... (#508806 - closed)

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Before	After

How to set up and validate locally

1. Enable wiki_comments feature flag.
2. Go to/create a new wiki page.
3. Comment on the page, mentioning yourself or other users.