Clean up schema for table oauth_applications

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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
• Polymorphic ? - owner_type always NULL or User
• Missing FK: owner_id -> users (?)
• Wrong data type: uid should be bytea, save 166 kB
• Wrong data type: secret should be bytea, save 166 kB
• Duplicate data: redirect_uri repeated
• Duplicate data: scopes could be an enum or 4-byte integer
• redirect_uri sometimes multi-line (?)