Updates .gitlab/ci scripts on 12-4-stable

What does this MR do?

Related to https://gitlab.com/gitlab-com/gl-infra/infrastructure/issues/8727#note_262807975

Updates .gitlab/ scripts on 12-4-ee-stable. The following commits were cherry-picked:

• Don't run Review App jobs for backstage changes - Had to be picked otherwise the in preparation for the next commit
• Improve the only/except extendable jobs
• Allow failure for schedule:package-and-qa due to failure rate

Additionally, I had to modify the scripts to fix some failures. These modifications are wrapped on 76c02c45

Test MR on Security https://gitlab.com/gitlab-org/security/gitlab/merge_requests/15