Check import success for photos uploaded in background

[thielepaul]

https://github.com/photoprism/photoprism-mobile/pull/39/files#diff-20cd85847cb2347b954302c30b47f4f2R161

one possibility is to check if a thumbnail is available: https://demo.photoprism.org/api/v1/thumbnails/4c8047b246a89ea5de7d5ee73945020cd1b1278d/tile_50