update Plugins/Themes metadata from upstream API responses

[chuckadams]

Much like /download acts as a write-behind cache by copying the download file to S3, the /plugins and /themes routes should be able to do the same for their respective resources and insert/update the database entities appropriately. We still can't completely cut AspireSync loose -- we still need it to discover new plugins/themes and push updates for search -- but we can make it more of an optional optimization of AC for most purposes.