Add support for distinguishing rendered drawings vs photos in elevation images

[DIYglenn]

Currently the library treats all elevation images the same regardless of whether they are clean rendered drawings or real photographs.
It would be useful to have a way to distinguish between the two, for example via flags like image_type: render or image_type: photo.

This would allow the importer to prioritize for example rendered drawings for consistency in rack elevation views, while falling back to photos where no render exists. It would also help contributors know whether a photo contribution is still welcome even when a render already exists.

[harryajc]

I think that would be too confusing to have multiple different types of images defined for the same device and would also require the import script to be massively modified to support this which I don't think is feasible at least at the moment. I think the best we could do is to set a preference for what type of either render or photo is the prefered and then ask contributors to upload the preferred image type if available, and if not fall back to the other