Allow arbitrary sized thumbnails

Currently, there is a single, globally-set thumbnail size that all thumbnails must adhere to. We should instead allow arbitrary thumbnail sizes.

/download/thumbnails/9999/image.jpg – default thumbnail size (as before)
/download/thumbnails/9999/320x240/image.jpg – 320x240 thumbnail size

And so on.

• Gallery macro and thumbnail properties for images will need to support the sizings
• We will need to periodically flush the thumbnail cache directory, though, since we could have an arbitrary number of thumbnail files per attachment.
• As it is now, the aspect ratio of the original image should be preserved - the 320x240 thumbnail should be the largest image that will fit into a 320x240 rectangle while preserving the original aspect ratio.