Address the Primary Key Overflow risk for the deployments table - Step 1: Add new columns and sync data

Today the deployments table have deployable_type and deployable_id. These do currently refer only to ci_builds and should be migrated as well.

I propose that we introduce build_id and later drop polymorphic association with deployable_type and deployable_id.

TODO:

• Flesh out which columns/tables we're targeting here
• Flesh out a plan how to incorporate this into the PK migration