API response should include a URL for each image of an item

[Libisch]

(See #177)

API adds a URL to every picture in Pictures (to all Items, in order to provide front all URLs without needing to generate them).

Example for added PictureUrl in expected response:

[
    {
        "RightsDesc": "Full",
        "UpdateUser": "BH Online",
        "StatusDesc": "Completed",
        "UnitText1": {
            "En": "Today: Poland, in 1918 - 1939 Poland. Pre 1914 Russia, pre 1795 Poland.\nCourtesy of www.bagnowka.pl"
        },
        "UnitPeriod": [
            {
                "PeriodDateTypeDesc": {
                    "En": "Year",
                    "He": "שנים"
                },
                "PeriodDesc": {
                    "En": "",
                    "He": ""
                }
            }
        ],
        "main_image_url": "https://s3-us-west-2.amazonaws.com/bagnowka-scraped/full/17127fb4ee9ddfd5cc2d0bcb783c571620238d67.jpg",
        "DisplayStatusDesc": "Museum and Internet",
        "UnitTypeDesc": "Photo",
        "TS": "",
        "PeriodDesc": {
            "En": "",
            "He": ""
        },
        "Header": {
            "En": "Poland - draws postcards photos",
            "He": "null"
        },
        "thumbnail_url": "https://s3-us-west-2.amazonaws.com/bagnowka-scraped/full/17127fb4ee9ddfd5cc2d0bcb783c571620238d67.jpg",
        "UnitHeaderDMSoundex": {
            "En": "",
            "He": ""
        },
        "bagnowka": "True",
        "UnitId": "111111111118215",
        "_id": "111111111118215",
        "Pictures": [
            {
                "PictureUrl": "https://s3-us-west-2.amazonaws.com/bagnowka-scraped/full/cbb235f952efb99d143ab5c61f3f2cfb344cacbd.jpg",
                "PictureId": "cbb235f952efb99d143ab5c61f3f2cfb344cacbd"
            },
            {
                "PictureUrl": "https://s3-us-west-2.amazonaws.com/bagnowka-scraped/full/bfa952952b58642696d1bb8d09b77423acb808ce.jpg",
                "PictureId": "bfa952952b58642696d1bb8d09b77423acb808ce"
            },
            {
                "PictureUrl": "https://s3-us-west-2.amazonaws.com/bagnowka-scraped/full/9247652184a347611e401a42a2f3399c169a9889.jpg",
                "PictureId": "9247652184a347611e401a42a2f3399c169a9889"
            },
            {
                "PictureUrl": "https://s3-us-west-2.amazonaws.com/bagnowka-scraped/full/7e7545b2965e5d18aff338beda7cd59933d56bb9.jpg",
                "PictureId": "7e7545b2965e5d18aff338beda7cd59933d56bb9"
            },
            {
                "PictureUrl": "https://s3-us-west-2.amazonaws.com/bagnowka-scraped/full/746462243f6de9f23fee453d7345ee69a0300a8f.jpg",
                "PictureId": "746462243f6de9f23fee453d7345ee69a0300a8f"
            },
            {
                "PictureUrl": "https://s3-us-west-2.amazonaws.com/bagnowka-scraped/full/1ddb7692f9b3e9cf03a4e0f8c61c2a499ebac243.jpg",
                "PictureId": "1ddb7692f9b3e9cf03a4e0f8c61c2a499ebac243"
            },
            {
                "PictureUrl": "https://s3-us-west-2.amazonaws.com/bagnowka-scraped/full/c32b06c7ce81b0fc0a21792eb0e04e251156cd3c.jpg",
                "PictureId": "c32b06c7ce81b0fc0a21792eb0e04e251156cd3c"
            },
            {
                "PictureUrl": "https://s3-us-west-2.amazonaws.com/bagnowka-scraped/full/06cf187dccce57fa38d5fd1052ba563789ad74bc.jpg",
                "PictureId": "06cf187dccce57fa38d5fd1052ba563789ad74bc"
            },
            {
                "PictureUrl": "https://s3-us-west-2.amazonaws.com/bagnowka-scraped/full/25bbcbccade88b2b731e98c0a7cc33c4fa6cda89.jpg",
                "PictureId": "25bbcbccade88b2b731e98c0a7cc33c4fa6cda89"
            },
            {
                "PictureUrl": "https://s3-us-west-2.amazonaws.com/bagnowka-scraped/full/17127fb4ee9ddfd5cc2d0bcb783c571620238d67.jpg",
                "PictureId": "17127fb4ee9ddfd5cc2d0bcb783c571620238d67"
            }
        ],
        "Slug": {
            "En": "image_poland-draws-postcards-photos"
        },
        "related": [
            "place_belarus"
        ],
        "UpdateDate": "2017-05-21T11:04:28.000Z"
    }
]