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

Today the taggings define a polymorphic association for ci_runners and ci_builds. There are two polymorphic relations there taggable_ and tagger_. From what I see only taggable_* is today used.

We need to migrate taggable_id as in majority of cases it relates to ci_builds via taggable_id.

TODO:

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