Attachments data

[rimi-itk]

Note: This builds on #2 which should be reviewed first.

Adds attachment data to GET responses:

{
  …
  "data": {
    …
    "attachments": {
      "attachment_pdf": {
        "name": "Attachment (pdf)",
        "type": "pdf",
        "url": "http://os2forms.example.com/da/webform/os2forms/submissions/42/attachment/pdf/pdf.pdf"
      },
      …
    }
  }
}