Deleting a thumbnail doesn't delete alternative resolutions of that thumbnail

[solarissmoke]

Calling delete_thumbnails() on a KVStore entry only deletes the default thumbnail on disk - any thumbnails that were generated in alternative resolutions are not deleted.

This feels inconsistent with what someone would expect that method to do - if I ask it to delete all thumbnails, I would expect this to include all alternative resolutions as well.

I think a reasonable default behaviour would be to check THUMBNAIL_ALTERNATIVE_RESOLUTIONS and delete any associated files (if they exist) when the main thumbnail is deleted.

(There is still an edge case where this wouldn't necessarily work if THUMBNAIL_ALTERNATIVE_RESOLUTIONS has changed since the time the thumbnails were generated, but that is probably something that could be documented).