Add created_by: user_id to "Asset blobs"

[yarikoptic]

Establishing full accounting/provenance for changes in the archive would probably require a substantial work. To be able to track down and mitigate uploads by malicious users, we could at least quickly add accounting for the blobs uploads. It would IMHO be very much sufficient to be able to quickly

• traceback all content uploaded by a malicious user
• Dandisets records have history enabled, so we can trace down which dandisets were created by the user and remove them
• double-check if that content was not somehow used in other (legit) dandisets to not remove it. (use case: attacker upload 5% of bad content with 95% of legit, where legit stuff was reused in other dandisets later since we deduplicate).