Add a view that will clone a dataset in a background process

- You can clone an existing dataset by either:

    * specifying a `X-Shareabouts-Clone` header, or
    * including a `clone` querystring parameter.

  In either case, the value of the header/parameter can be one of three
  things:

    * the URL of the dataset to be cloned,
    * the id of the dataset to be cloned, or
    * a comma-separated pair of the dataset owner name and slug

  To clone a dataset, the authenticated user must have enough permission on
  the cloned object to read private and invisible data.