Add Slack notification for scheduled `package-and-qa` jobs

We need to notify the TAEs of the status when the schedule:package-and-qa job executes from a scheduled pipeline as apart of https://gitlab.com/gitlab-org/quality/team-tasks/issues/216.

We discussed in the 2019-09-18 Engineering Staff meeting to create a channel qa-master for these notifications which might be a good start but open to other ideas.

!17237 (merged) contains the details for changing the schedule:package-and-qa execution logic.

@gl-quality/eng-prod I'd prefer to get this done early in 12.4. Could someone pick this up?

• Create channel qa-master
• Add Slack notification scheduled:package-and-qa !17796 (merged)
• Add link to failed gitlab-qa pipeline to Slack notification (Slack discussion: https://gitlab.slack.com/archives/CNV2N29DM/p1569999379008700)