Make photo thumbnail size preference configurable

Problem/Motivation

Currently the default order of photo thumbnail size to fetch from AVPortal is Medium - High - Low. In some contexts the medium thumbnail size is not ideal, and it would be nice if this could be changed to use the higher resolution.

Proposed resolution

This could be improved by either making this a configurable preference, or just changing the default order to use the high res thumbnail image (sites then can still apply their prefered image style on the thumbnail for display, but on a higher res source image).