Summary:
After an incomplete import, any GristHidden_* tables will show up in the page
list, but may not be removable if there is only one non-hidden table remaining.
Such tables should still be removable in this case.
Test Plan: Added a test case
Reviewers: georgegevoian
Reviewed By: georgegevoian
Differential Revision: https://phab.getgrist.com/D3058
