Improve UploaderInfo with some JOINs

[ro-tex]

f/u

Definitely not a requirement for this PR but I wonder if PJ could turn this into some fancy query which fetches all uploads for a skylink id and then joins them with their users where possible. So that all of that happens with a single query.

Originally posted by @ChrisSchinnerl in #182 (comment)

f/u Might be a tiny bit more efficient to use a map to pointers to avoid this dereference here. Because the ID, Email, Sub and StripeID are later copied again anyway.

[ro-tex]

https://www.mongodb.com/docs/manual/reference/operator/aggregation/lookup/
https://hevodata.com/learn/mongodb-joins/