[schema] Use TZDatetime for creation_date and modification_date
No work has been done for form fields/widgets since creation_date /
modification_date are by default not editable through the default UI. One may
want to add such feature at some point.
Time will be displayed as UTC, let the end-application decide otherwise by
customizing the tzdatetime printer if desired.
Closes #4848923
[server] fix SQLite patch to ensure TZDatetime are properly typed
NB: at this point, the TZ info is completely ignored. It will be
properly handled in later commits.