Stage 3: Implement post-deployment notifications

As part of &608 (closed), we want to have individual pipelines for post-deployment migrations. The first step for this is being worked on #2107 (closed) and it was discovered that we need specific post-deployment notifications when the post-deploy migrations fail gitlab-org/release-tools!1630 (comment 772521448).

Later on we could re-use these notifications for &585 (closed)

Proposal

Build a new notification that:

1. It's triggered if the post-migrations pipeline fails.
2. Send a message to Slack stating that the post-migration failed.
3. Link to the pipeline that triggered the post-deploy migration.