blobs_size to /stats

[yarikoptic]

#298 fixes the total size reported for the archive to be the "interesting" one - the amount of (unique) data addressable from draft or published dandisets. But I wonder if we should also add blobs_size which would be computed directly from blobs table as the amount of blobs known so we could assess how much should be picked up by GC

[waxlamp]

Closing this in favor of #263 and #177.