Clean up schema for table keys

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

• Wrong data type: last_used_at should be timestamptz
• Missing FK: user_id -> users (493488 entries violate this rule)
• Missing constraint: key NOT NULL
• Missing constraint: fingerprint NOT NULL (1 entry violates this rule, id 666)