Enable notifications for deployments in Chat Notification Services

Description

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.

Proposal

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.

Links / references

/app/models/project_services/chat_notification_service.rb