Add sharding key to labels

labels table already has a sharding key definition (group_id, project_id). But we recently discovered we can have records that don't have any of those 2 columns set. These records work as default label templates for all new projects.

Related discussion in &16534 (comment 2517549528)

We probably need to add a new sharding key column organization_id to the labels table so it is assigned for cell wide records

TODOs

• Remove fallback in admin labels controller and spec app/controllers/admin/labels_controller.rb and spec/requests/admin/labels_controller_spec.rb Moved to Cleanup template labels after backfill is complete (#563889) as cleanup steps can only be performed in 18.6