Alert Management alerts Slack Notifications

What does this MR do?

Adds Alert Management alerts as a Slack Notification event.

When a new alert is triggered, a Slack notification will be sent to the specified channel. This message provides detail of the alert & links back to GitLab to see full details.

Subsequent events for an alert do not produce a slack notificaiton.

Feature flag: This is currently behind a feature flag. Follow up: #220415 (closed)

Screenshots

Settings	Slack message

Database outputs:

rake db:migrate

 rake db:migrate:up VERSION=20200526013844
== 20200526013844 AddAlertEventsToServices: migrating =========================
-- add_column(:services, :alert_events, :boolean, {:default=>false, :null=>false})
   -> 0.0157s
== 20200526013844 AddAlertEventsToServices: migrated (0.0225s) ================

rake db:rollback

rake db:migrate:down VERSION=20200526013844
== 20200526013844 AddAlertEventsToServices: reverting =========================
-- remove_column(:services, :alert_events)
   -> 0.0010s
== 20200526013844 AddAlertEventsToServices: reverted (0.0073s) ================

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry behind feature flag see #220415 (closed)
• [-] Documentation (if required) behind feature flag see #220415 (closed)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• [-] Separation of EE specific content

Availability and Testing

To test, setup a slack integration a Slack Workspace you have access to. Setup the Slack notifications, and ingest some alerts.

Part of #216326 (closed)