Add a new Ci::ApplicationRecord that all CI models should inherit from.
This also replaces the usage of the previous Gitlab::Ci::Model.

**app/models/ci_platform_metric.rb:**

- Implements BulkSafeInsert
- Changes validation to allow "" for platform_target
- Changes validation to check for integer count
- Changes validation to check for count > 0

**db/structure.sql:**

- Adds constraint for count > 0

**db/migrate/20200908183231_add_check_positive_
constraint_to_ci_platform_metrics.rb:**

- Adds migration for constraint count > 0

**db/schema_migrations/20200908183231:**

- Migration artifact

**spec/models/ci_platform_metric_spec.rb:**

- Updates spec tests for changes to model

**changelogs/unreleased/ci-platform-metrics-bulk-insert.yml:**

- It's a changelog

**app/models/ci_platform_metric.rb:**

- New model.

**db/structure.sql:**

- Updated structure following migration.

**db/migrate/20200820204041_create_ci_platform_metrics.rb:**

- Migration.

**db/schema_migrations/20200820204041:**

- Migration artifacts.

**spec/factories/ci_platform_metrics.rb:**

- New builder for this model.

**spec/models/ci_platform_metric_spec.rb:**

- New spec tests for this model.

**changelogs/unreleased/ci-platform-metrics.yml:**

- Required changelog addition.