Write thumbnail sizes to AssetApplicationMetadata table

[donaldgray]

This ticket is created in response to RFC #779, which was initially written to address requirement in #631.

As mentioned in RFC we need a new AssetApplicationMetadata table to store various bits of metadata about an asset. This will initially store thumbnailSizes, duplicating what is saved in s.json (we will keep writing this file to S3).

This ticket is then:

• Create new DB table
• Update engine image processing logic to write thumbnail sizes to this DB, in addition to saving same values to S3.