Production db snapshots don't expand superblobs

[bhearsum]

eg, SystemAddons for Firefox 63.0* on release gets Superblob-26b1ac78303807dfeebb87f39cbdfd1a8a095d7fedc5d449ac6c409962a6cec8, and that's in the snapshot. It has content of

{
"blobs": [
"[email protected]"
],
"name": "Superblob-26b1ac78303807dfeebb87f39cbdfd1a8a095d7fedc5d449ac6c409962a6cec8",
"schema_version": 4000
}

[email protected] isn't in the snapshot.

(Imported from https://bugzilla.mozilla.org/show_bug.cgi?id=1510863, filed by @nthomas-mozilla)