Scan source code for security issues

This is about static analysis of security vulnerabilities, not scanning a running application, for example.

• Use open source libraries to scan code
• Use CI to run scanner
• Make it part of Auto DevOps (like code quality and tests)
• Detect language/framework and run matching scan tool

cc @markpundsack

Based on https://gitlab.com/gitlab-org/gitlab-ce/issues/32000#note_41469199.

Maybe duplicate of https://gitlab.com/gitlab-org/gitlab-ee/issues/2592