[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

addressbook
basket
blog
book
calendar
comment
company
email
file
folder
i18ncontent
keyword
link
mailinglist
person
tag
timeseries
vcsfile