Clean up schema for table notification_settings

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From https://gitlab.com/gitlab-com/infrastructure/issues/1709 (private to GitLab employees):

• Wrong data type: created_at should be timestamptz
• Wrong data type: updated_at should be timestamptz
• Missing FK: user_id -> users (17 entries violate this rule)
• "Polymorphic" table prevents regular FKs, use triggers or change to independent columns or non-polymorphic tables
• Duplicate data: source_type could be enum or 4-byte integer, save 20 MB
• Missing trigger for FK check: source_type = 'Namespace', source_id -> namespaces.id
• Missing trigger for FK check: source_type = 'Project', source_id -> projects.id (3 entries violate this rule)
• Missing constraint: CHECK (source_type IN ('Namespace','Project'))
• Duplicate index: index_notification_settings_on_user_id - notification_settings (user_id), already indexed
• Duplicate data: events column is structured data (JSON?) stored as text, lots of repeated data, avg tuple width is ~250 bytes, 200+ from events column