Returning full Asset in Manifest response

[donaldgray]

Follow on from #173 (which is returning the asset-id only) this ticket is to flesh out the returned asset result to include the full DLCS asset.

To do this call /allImages, specifying the ids of the assets contained in the manifest. Note that /allImages has a maximum number of ids so we may need to make multiple requests.

These should be returned in the same order as they are in the canvas_paintings table.

If an asset could not be found, return a "placeholder" asset in the result set, with just an "id" and "error"

[donaldgray]

Note - this ticket mentions using /allImages to get details of all assets. dlcs/protagonist#932 introduced a new endpoint where all assets could be fetched for a batch, which is a possibility.

Consider trade off of number of calls to be made, size of payloads etc.