[TASK] Use ISO-8601 dates for display and processing

This changes the internal date processing for FormEngine and DataHandler
to use ISO-8601 dates including a proper timezone. Dates are converted
to ISO-8601 as early as possible and converted back to UNIX timestamps
as late as possible.

As before, the database values are always values in the server’s
timezone, interpreted as UTC. Also, the client side inputs are
interpreted as UTC.

The main advantage is that once and for all we get rid of the timezone
issues that may potentially arise if the server and client use different
timezones. Additionally, the values are human readable (which is of
course not so much of an issue for hidden fields) and we can directly
use Moment.js for all heavy lifting.

Another big advantage is that we can make the date formats configurable
and decouple displayed dates and internally stored dates (by putting the
Moment objects to the input fields).

Change-Id: I3461915c2beaa96cd29c52c60e65a2e5005065b7
Resolves: #77702
Releases: master
Reviewed-on: https://review.typo3.org/47124


Reviewed-by: Susanne Moog <susanne.moog@typo3.org>
Tested-by: Susanne Moog <susanne.moog@typo3.org>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Oliver Hader <oliver.hader@typo3.org>
Tested-by: Oliver Hader <oliver.hader@typo3.org>