Fix Dropzone issue changing existing photos.

I used the Dropzone source code to come up with what was missing when
adding existing files (we were previously reducing maxFiles which had
much the same effect unless you tried to remove/add when it got quite
confused); after doing that, I then discovered a PR on Dropzone which
has the same fixes: dropzone/dropzone#2003

web/cobrands/fixmystreet/fixmystreet.js

@@ -887,15 +887,17 @@ $.extend(fixmystreet.set_up, {
             if (!f) {
                 return;
             }
-            var mockFile = { name: f, server_id: f, dataURL: '/photo/temp.' + f };
+            var mockFile = { name: f, server_id: f, dataURL: '/photo/temp.' + f, status: Dropzone.SUCCESS, accepted: true };
             photodrop.emit("addedfile", mockFile);
             photodrop.createThumbnailFromUrl(mockFile,
                 photodrop.options.thumbnailWidth, photodrop.options.thumbnailHeight,
                 photodrop.options.thumbnailMethod, true, function(thumbnail) {
                     photodrop.emit('thumbnail', mockFile, thumbnail);
                 });
             photodrop.emit("complete", mockFile);
-            photodrop.options.maxFiles -= 1;
+            photodrop.files.push(mockFile);
+            photodrop._updateMaxFilesReachedClass();
+            prevFile = mockFile;
         });
       });
     });