Automatically create thumbnail

[stephenwf]

When a user add a painting annotation with a single image, we should have enough information to automatically generate a thumbnail.

We could either:

• Use a side-effect, similar to the resizing canvas, that will pick a thumbnail automatically
• Add some UI to select a thumbnail as a second step

The first option would remove the options for customising the thumbnail, but would be out-of-mind for the user, with thumbnails appearing automatically. We can also have some simple detection for DLCS hosted to look for a /thumbs/ endpoint too.

The second option would be more of a rich experience, allowing the user to:

• Choose from the available sizes in a dropdown
• Specify a custom size if supported (?)
• Could look for DLCS /thumbs/ and prompt to add that (with choice of size too).

The second option could also be an editor to enhance the first option.

[tomcrane]

(see also #129)

[stephenwf]

Thinking about this more generally, it also overlaps with the "get thumbnail at size" logic in a way, although limited to just a single image resource.

• What are the available fixed sizes?
• Does the image service support arbitrary widths/heights
  • Is there a defined max area?
  • If no max area, what is a safe size (tile size?)
• Is this a level0 image?
  • Is there a level0 full tile that can be used as a thumbnail

Creating a list of "candidates" from an image + service, and then presenting these as a UI for the user to select from would be an end goal for an editing interface.

For now, we can simply look at the sizes.

[tomcrane]

For the DLCS-specific scenario, even if the Manifest Editor didn't know that there might be a matching /thumbs/ path (it wouldn't know if sitting behind a proxy like Wellcome for example, where the public path structure is simpler), just using the advertised sizes would be almost as efficient.