feat: provide slideshow media for images

[mgogoulos]

Description

This is required for #1106

provides the following structure in the API for media, if media_type is image

"slideshow_items": [
        {
            "poster_url": "/media/original/thumbnails/user/admin/5033538082024d149e72df9bfe89e995.DSC07897.JPG_yvDV18F.jpg",
            "url": "/view?m=RFMfIrCEp",
            "thumbnail_url": "/media/original/thumbnails/user/admin/5033538082024d149e72df9bfe89e995.DSC07897.JPG.jpg",
            "title": "title x "
        },
        {
            "poster_url": "/media/original/thumbnails/user/admin/35d1d110f3f246c28c5c61c8fa4192be.DSC07896.JPG_GvqGmSY.jpg",
            "url": "/view?m=iuOtTFJNx",
            "thumbnail_url": "/media/original/thumbnails/user/admin/35d1d110f3f246c28c5c61c8fa4192be.DSC07896.JPG.jpg",
            "title": "title xy"
        },
    ],

If media_type is not "image", then this is []