Clean up schema for table appearances

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: title NOT NULL (perhaps Unique?)
• Missing constraint: description NOT NULL
• Missing constraint: description_html NOT NULL
• Missing constraint: created_at NOT NULL
• Missing constraint: updated_at NOT NULL