Add iid to ml_candidates

In table :ml_candidates, the column iid refers to a uuid that is created by the MLFlow client. This breaks convention with the rest of the gitlab's codebase and should be changed. ml_candidates would also benefit from a proper iid scoped to the project.

All MRs on 15.11:

• Rename ml_candidates.iid to eid (!115356 - merged)
• Adds scoped internal id to Ml::Candidates (!115401 - merged)
• Backfill Migrations for ML::Candidate.internal_id (!115971 - merged)
• Refactors to use Ml::Candidate.internal_id inst... (!116085 - merged)