Add better mocking for stores

Created by: camdencheek

This tracks the progress towards implementation of #26129.

Game plan:

• Convert store structs to interfaces and generate mocks for each store
• Mint database.DB type that has methods to return specific store types
• Move store doc comments from implementing structs to interface
• Replace global mocks with injected mocks
• Replace uses of dbutil.DB with database.DB (in progress)
• Replace uses of database constructor functions with database.DB methods (in progress)
• Figure out how to handle enterprise-only stores in a mockable way
• Change database.NewDB() to take a *sql.DB rather than a dbutil.DB
• Update transaction code to avoid casing on the concrete type of a dbutil.DB
• Remove uses of dbconn.Global since it can't be mocked: done

See progress in a code insights dashboard here