Add sharding key to label_links table

Mario Celirequested to merge
514599-add-label-links-sharding-key into master

What does this MR do and why?

Add column and start setting it on the ActiveRecord model. First step into discovering why we have so many orphaned records. If orphaned records have a namespace_id set after this change, it means that records are becoming orphans after they are initially created

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Related to #514599 (closed)

Edited by Mario Celi

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