Enable notifications for deployments in Chat Notification Services
Currently, the Slack/Mattermost notifications (and chat notifications in general) settings allow for events such as Push, Issue, Merge, Note, Pipeline, etc, but it does not have a specific event for new deployments.
Create a new notification event for deployments (either manual or automatic, successful or failed), for integration with the Slack notifications webhook. This builds on https://gitlab.com/gitlab-org/gitlab-ce/issues/42692 (also scheduled for the same release) and requires either no or very limited additional configuration, but ensures we complete the loop in tying the webhook to chat notification services.
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