Summary:
Previously, when a table was deleted, ref/reflist columns pointing at that table were deleted as well. This diff changes that, converting the columns to a suitable type instead.
See here for discussion and decisions: https://grist.slack.com/archives/C069RUP71/p1686060034848139
Test Plan: Added Python tests for most cases, and a DocApi test for where Python has to call JS code to convert the values.
Reviewers: paulfitz
Reviewed By: paulfitz
Differential Revision: https://phab.getgrist.com/D4011