Rating selection tweaks

[russss]

I think before we start importing many more images, we need to improve the way images are selected for rating. Some quick thoughts:

• By default, we should probably prioritise newer images for rating, as those will tend to be more interesting (especially once we have semi-realtime importing).
• You should be able to rate a specific mission if you want. This is especially important when we import a new mission and we want to bubble up some good images quickly.
• Rating lots of similar images is dull and potentially confusing. We don't have good similar-image detection yet, but we could possibly de-prioritise rating images in a group which you've already rated an image from.