Switch to json encoded meta data

Instead of parsing a comma separated list of photo urls, we retrieve a
json array of meta data (url, title, size).

elm.json

@@ -6,16 +6,17 @@
     "elm-version": "0.19.1",
     "dependencies": {
         "direct": {
+            "NoRedInk/elm-json-decode-pipeline": "1.0.1",
             "elm/browser": "1.0.2",
             "elm/core": "1.0.5",
             "elm/html": "1.0.0",
             "elm/http": "2.0.0",
+            "elm/json": "1.1.3",
             "elm/random": "1.0.0"
         },
         "indirect": {
             "elm/bytes": "1.0.8",
             "elm/file": "1.0.5",
-            "elm/json": "1.1.3",
             "elm/time": "1.0.0",
             "elm/url": "1.0.0",
             "elm/virtual-dom": "1.0.3"