Submitting translations removes quality check submissions

[julen]

When a unit contains submissions related to changes in quality checks (mute/unmute), these are properly reflected in the timeline.

However, when a new translation is provided (including an empty string), quality checks for that unit are deleted, and with that, due to the ON DELETE CASCADE behavior, the Submissions for the unit are deleted as well. After this, the timeline no longer reflects the past changes to muting/unmuting quality checks.

Changing the ON DELETE behavior to SET NULL wouldn't help because we need to have access to the quality check name, which is in the now-removed QualityCheck. We can alternatively denormalize the check name into Submission, but we should be aware that that requires increasing the size of the submissions table for a limited use case, plus schema and data migrations.