Sign Instance
GET /data/api/signs/159/?format=api
http://labasi.acdh.oeaw.ac.at/data/api/signs/159/?format=api", "sign_name": "AM", "abz_number": "170", "meszl_number": "309", "image_1": "http://labasi.acdh.oeaw.ac.at/media/sign_img/sign_170.bmp", "image_2": null }{ "url": "