Make all SystemHook trigger events configurable

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Problem to solve

For system hooks, certain events are always triggered and can't be configured in the admin area.

Example: The :failed_login event triggered from https://gitlab.com/gitlab-org/gitlab/blob/9ed55b6da2451b66739482c2a01996ad66c51b36/lib/gitlab/auth/blocked_user_tracker.rb#L17

Proposal

• Find all event names that are used to trigger system hooks.
• Assign them to one of the existing *_events columns on web_hooks, or add new ones.
• Refactor the existing code to only trigger these events if the corresponding *_events column is enabled.
• Make sure these events are correctly exposed on the admin form and API.
• Update docs.

Related issues

In gitlab-com/gl-infra/production#5501 (closed) we had to use a workaround to disable a misbehaving system hook on gitlab.com. With this issue we could instead just disable all trigger events.