[API-216] Migrate /v1/users/:userId/collectibles

[schottra]

Extremely basic endpoint. We're just fetching the raw data from the row and returning it back. Fun quirks:

• I named the column data originally, so it's data.data in the response.
• Similar to a few other endpoints, we don't 404 on not found, just return null. Can't guarantee client won't error if we break from that behavior.

[raymondjacobson]

nice seems good!