Make 3 key improvements to reduce overall duration time in GitLab CI pipelines

This issue tracks a FY20-Q3 key result for the Engineering Productivity team (gitlab-com/www-gitlab-com#4853 (closed)).

Improvements

1. Reduce complexity: !15121 (merged)
2. Use needs (DAG): gitlab-foss!32085 (merged) + gitlab-foss!32366 (merged)
3. Use Pipelines for Merge Requests with change detection: !15761 (merged)

Timeline of improvements

• 2019-08-26 2:39 PM UTC: Use the 'needs' keyword for 'package-and-qa' job. This should save up to 26 minutes for QA branches. gitlab-foss!32085 (merged)
• 2019-08-27 4:24 AM UTC: Reduce the complexity of our CI configuration. gitlab-foss!31728 (merged) / !15121 (merged)
• 2019-09-05 5:03 PM: DAG changes for setup-test-env and review-deploy. gitlab-foss!32366 (merged)
• 2019-09-17 1:39 PM: Use Pipelines for Merge Requests with change detection. !15761 (merged)