Clean up schema for table application_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 constraint: created_at NOT NULL
• Missing constraint: updated_at NOT NULL

We also need to add a constraint (in the DB) to enforce only a single row being allowed in this table. Done as of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/14718