Improve image cache purge to include failover assets

Mateusz:
broken [Fastly image cache busting](https://frontend.gutools.co.uk/images/clear)? Despite gazillions of clicks on Clear yesterday and today, this
https://i.guim.co.uk/img/media/76b38f30b2228a9db72ada8a39bd279b8b3db4f4/0_224_6720_4032/master/6720.jpg?width=620&dpr=2&s=none
is still online.

David:
As far as I can tell we are a victim of our own cleverness with the fallback mechanism. The cache purger has purged https://i.guim.co.uk/img/media/76b38f30b2228a9db72ada8a39bd279b8b3db4f4/0_224_6720_4032/master/6720.jpg (with all query params)
and https://media.guim.co.uk/76b38f30b2228a9db72ada8a39bd279b8b3db4f4/0_224_6720_4032/master/6720.jpg

But it has not decached the non-master sizes (e.g. https://media.guim.co.uk/76b38f30b2228a9db72ada8a39bd279b8b3db4f4/0_224_6720_4032/1000.jpg). These are used when Fastly IO "fails", which includes returning a 404.

The fix would be to update the purge to include all versions in the [media.guim.co.uk](http://media.guim.co.uk/) bucket.