Strip missing images from the dataset

[asnare]

Some of the images we're using from Flickr have been removed by the original author. On load these are replaced with an "image has been removed" placeholder.

We should drop these from our dataset. To detect them:

1. Request the image URL.
2. If it's been removed a 302 redirect is issued to https://s.yimg.com/pw/images/en-us/photo_unavailable.png