Merge branch 'backstage/gb/populating-pipeline-refactoring' into feature/gb/variables-expressions-in-only-except

* backstage/gb/populating-pipeline-refactoring: (860 commits)
  Add specs for extracted pipeline-related methods
  Bring seeds size method back to CI/CD pipeline class
  Improve YAML processor specs and remove unused code
  Fix static analysis and tests related to YAML processing
  Add specs for pipeline chain that builds stages and jobs
  Do not select included stage seeds in populate chain
  Decouple YAML processor from pipeline objects
  Move policy related specs our of YAML processor tests
  Fix rubocop offense in pipeline build seed class
  Add a guard clause to avoid creating empty stages
  Evaluate only/except policies outside of YAML processor
  Make it easier to test pipeline stage seeds
  Reduce stage seeds coupling between pipeline and YAML
  Add specs for chain element that populates a pipeline
  Populate pipeline with objects before creating it
  Remove legacy service for creating pipeline stages
  Decouple pipeline stage seeds from building environments
  Support building all pipeline resource before saving
  Introduce pipeline build seeds
  Start building abstraction over pipeline seeds
  ...

Conflicts:
	lib/gitlab/ci/yaml_processor.rb
	spec/lib/gitlab/ci/yaml_processor_spec.rb