Summary:
When fields or sections were being removed from Grist documents, any
layout specs that referred to them weren't being updated to no longer
do so. This mismatch was causing various buggy scenarios to manifest
in cases where the stale ids were being reused. We now automatically
update any affected layout specs whenever fields or sections are
removed.
Test Plan: Python tests.
Reviewers: jarek
Reviewed By: jarek
Differential Revision: https://phab.getgrist.com/D4302