277 dataset thumbnail 404 not found broken link after file deletion

[longshuicy]

Description

First to reproduce this issue, see the latest comment I made at the issue #277

In short, deleting a file used as thumbnail IS FINE :-p.

Only when that image (used as thumbnail) is moved to a folder --> then got deleted causes issues. The reason is being that images inside of the folder are NOT used as thumbnail candidates at the moment; however when moving an existing thumbnail to the folder failed to update that dataset/collection.

To fix that I added additional logic when move file to folders

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To test try below steps:

1. Upload 2 images to the root dataset - you should see the first image being used as thumbnail of the dataset
2. Create a folder
3. Move one of the image to the folder - you should see the thumbnail of the dataset changed
4. Move the other image to the folder - you should see the dataset is without thumbnail anymore
5. Delete folder/ or delete images inside of the folder - should not be any broken links

you could repeat the above step with dataset within collections, you should see similar results for the collection thumbnail.

Review Time Estimate

• Immediately
• Within one week
• When possible

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My change requires a change to the documentation.
• I have updated the CHANGELOG.md.
• I have signed the CLA
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.