[fix] ensure the default display size for a thumbnail (currently the full image) does not crash (closes #77)
[evol] added helper methods for ArtworkStep<Image|Thumbnail>
[fix] tries to regen missing thumbnail
[cosmetic] added position number to artwork_step_images, only visible in the admin pages, to be able to change order for steps made the same day (closes #16)
[evol] ArtworkStepImage: added helper method to get a thumbnail by size
Initial version