Add helper api method for DELETE requests

[davemcorwin]

• Add delete method to API:

export function delete (url, body) {
  return http(url, {
    ...OPTIONS,
    body,
    method: 'DELETE',
  })
}

• Add DELETE to list of CSRF methods in http:

const CSRF_METHODS = ['PATCH', 'POST', 'PUT', 'DELETE']