Bulk populate AssetApplicationMetadata thumb sizes

[donaldgray]

When AssetApplicationMetadata is released it initially won't have any thumb size rows, these will be added gradually when assets are ingested.

We need a bulk script to:

• Walk through all available Image assets
• Check if there is a corresponding AssetApplicationMetadata row containing thumbs sizes, if so abort. If not:
• Read the sizes from s.json and add to DB.

This script should be scalable to allow multiple processing running at the same time without stepping on each others toes (see https://github.com/dlcs/test_scripts) for examples.

This is blocked by #787