Date localization

[Marius Gedminas]

SchoolTool currently stores strftime-style date formats such as '%d %B, %Y' in
person preferences. This will result in incorrectly translated month names if
the system's locale does not match user's language (which may be determined from
the browser-sent headers).

We could use zope's locale time formatters instead of strftime (or in addition
to). See ILocaleDates.getFormatter. There are four options then: short,
medium, long and full, so it is meaningful to keep the preference, but change
the set of choices.