Add an endpoint to make it easier to replace thumbnails

[manthey]

This also allows attaching data to large_image files.

The core of this work is

datafile = Upload().uploadFromFile(
    io.BytesIO(data), size=len(data),
    name='_largeImageThumbnail', parentType='item', parent=item,
    user=user, mimeType=mimeType, attachParent=True)
datafile.update({
    'isLargeImageThumbnail' if thumbnail is not False else 'isLargeImageData': True,
    'thumbnailKey': key,
})
datafile = File().save(datafile)

where data is the binary data to store, item is the girder item, and key is the standardizes key for the thumbnail/data (e.g., {"height":100,"width":160}, where order and spacing matter).

The new endpoint replaces an existing thumbnail or adds if it doesn't exist.