Bug 29718: (QA follow-up) Add optional offset handling

Bug 29718: (QA follow-up) Add optional offset handling

ISO8601 optionally supports timezone offsets. If a timezone is ommited,
we should assume 'local' time (Which in our case is instance
configuration time), if we are passed 'Z' or 'z' we should set timezone
to 'UTC', otherwise we should set the timezone to the passed offset.

Whilst ISO8601 does NOT support AM/PM, I left that intact as it has
historical relevance in Koha and we share the ISO parsing with SQL type
parsing amongth other things.

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>