Fix user_stitching bug with differing column lengths

[emielver]

Describe the bug

The stitched_user_id column is being created as a varchar column of the same length as device_user_id. If the user_id is longer than the device_user_id then this leads to errors since we later update the value of stitched_user_id to that of user_id.

Steps to reproduce

Create dummy data where the domain_userid is a VARCHAR(6) and the user_id is VARCHAR(12).