Bug 12669: Centralize the timezone handle into Koha::DateUtils

Bug 12669: Centralize the timezone handle into Koha::DateUtils

This patch adds unit tests for the previous changes and centralize the
timezone handle into the Koha::DateUtils module.
Like that the behavior will affect all date manipulations using this
module (should be all dates in Koha).

Signed-off-by: Chris Cormack <chris@bigballofwax.co.nz>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>
(cherry picked from commit f82064286d3bc97031ad784901716a7a327839bc)
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>