Our existing thumbnailer code was pretty limited, and only allowed people to

pass in a certain image size as a string. This change improves it, especially

for the case where it's being used from Python code. Callers can now pass in

the thumbnail size as a 2-tuple of width and height. If the height is omitted,

it will be calculated from the width to preserve the aspect ratio of the image.

Used this in conjunction with my big file attachment redesign.