Change user_ids from integers to strings

[anandaroop]

While investigating https://github.com/artsy/gravity/issues/11189 I noticed something that hadn't caught my attention before.

In the db schema for Rosalind, the user_id field in the BatchUpdate model is defined as an integer, whereas it would need to be a string to accommodate the BSON ids coming from Gravity.

Since it's been defined as an integer the BSON ids are being truncated in the database, e.g.:

"548102be7261692d4ca00500" → 548102

(This is exactly analagous to the fix in 69ee68e, and I should have noticed that user_id would have been affected too)

I think the solution is pretty simple. We will

• migrate the data type for that column
• then replace the truncated IDs with the full ones

Luckily only a handful of users account for the ~200 batch updates that have been performed so far, so coming up with the mapping shouldn't be too hard.