LP1840782 locale/timezone aware date display formatter

LP1840782 locale/timezone aware date display formatter

Adds a method to the DateUtil class which can generate a human-friendly,
timezone and locale-aware display value for a date.  It supports canned
date and time formats, following the options described here (see
dateStyle and timeStyle):

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/DateTimeFormat

Adds a tool to the sandbox for generating date strings based on locale,
time zone, date format, and time format.

As noted in the sandbox page and within the utility code, Firefox
support for canned dateStyle and timeStyle values is pending:

resolution of https://bugzilla.mozilla.org/show_bug.cgi?id=1557718.

In the meantime, the code generates locale-aware "short" date and time
values in Firefox.

Signed-off-by: Bill Erickson <berickxx@gmail.com>