Add FK on events.personal_namespace_id

Krasimir Angelovrequested to merge
462801-add-fk-on-events-gitlab-com into master

What does this MR do and why?

Add FK on events.personal_namespace_id for GitLab.com only. Already done for self-managed with !165591 (merged).

Unfortunately, !164979 (merged) was merged and deployed before the FK for GitLab.com was created. To avoid validation failling, we need to cleanup any records that may point to personal namespaces that were deleted. Since we started writing to events.personal_namespace_id just recently, and only when there is no project or group associated, doing this in post-deployment migration does not take too long.

See #462801 (closed).

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Related to #462801 (closed)

Edited by Krasimir Angelov

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