Summary:
Date filter was not taking timezone correclty into account, which was
causing to wrong-inclusion and wrong-exclusion of dates near the
bounds.
Diff fixes that, it also bring little refactoring that hopefully clarifies things a little.
Test Plan: Includes brand new test for `app/common/ColumnFilterFunc`.
Reviewers: jarek
Reviewed By: jarek
Differential Revision: https://phab.getgrist.com/D3763