Summary:
JSON import logic was creating columns of type Int when JSON contained integral
values. This causes errors with large errors (e.g. millisecond timestamps), and
Numeric is generally the more convenient and common default.
Test Plan: TBD
Reviewers: jarek, alexmojaki
Reviewed By: jarek, alexmojaki
Subscribers: jarek, alexmojaki
Differential Revision: https://phab.getgrist.com/D3339