Add QA rake and notifier classes

What does this MR do?

Introduces new Quality classes:

• A rake task to trigger quality pipelines on quality/staging and quality/gitlab_qa
• A notifier that sends Slack notifications when QA fails.

Context for the QA Notifier

As part of gitlab-com/gl-infra/delivery#1490 (closed), quality jobs will be extracted into the coordinated pipeline, as such, besides the current notifications, we also need to deal with notifications when fails:

When deployment and QA are both successful	When deployment fails	When deployment succeeds and QA fails
Notification stays the same	Notification stays the same (QA is not executed in this case)	New notification

Re-using the "deployment failed" notification for QA failures is not feasible for two reasons:

1. Deployment failed notification links to the deployer pipeline, not to the coordinated pipeline.
2. CI configuration gets complicated, I couldn't find a way to execute notify_failure:* when a deployment and QA fails; e.g. when a deployment to staging fails (notify_failure:gstg job is executed here), it's retried successfully, and then QA is executed and fails, in this case, another failure notification needs to be sent, but we can't re-trigger notify_failure:gstg since it was already executed.

Author Check-list

• [-] Has documentation been updated?