fix #1284, fix #1313 -- fix dates in new_post

This is #1284 and #1313.

This effectively reverts the new locale-aware date format feature, as introduced in #1156, and forces one, internationally recognized format: ISO 8601.  However, it keeps the "UTC/UTC+01:00" timezone representation format as the default: it’s more user friendly than the representation used in the actual ISO standard (Z/±00:00); however, FORCE_ISO8601 can still be used if you want even more ISO 8601 in your life.  This format is very similar to what v6.4.0 used, with the only exception being the ``/`` replaced with a ``-``.

There are no changes in the parsing.  This means:

* dates generated by v6.4.0 are still accepted and work without changes (just like they did in v7.0.0)
* dates generated by v7.0.0 may or may not work (see #1284), and might need to be corrected (dd/mm/yyyy in particular)

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This might be of interest to: @ralsina, @dlitvakb, @humitos, @remram44, @ishamf, @ccanepa.

Signed-off-by: Chris “Kwpolska” Warrick <kwpolska@gmail.com>