Bug 16911: Koha::Patrons - Move ExtendMemberSubscriptionTo to ->extend_subscription

Bug 16911: Koha::Patrons - Move ExtendMemberSubscriptionTo to ->extend_subscription

This patch moves the code from C4::Members::ExtendMemberSubscriptionTo
to Koha::Patron->extend_subscription.

The expected behavior is:
When a new patron is created, the enrolment period defined for the
patron category is used unless an enrolment period date is defined. In
that case, this date is used.
When an account is renewed, the pref BorrowerRenewalPeriodBase is used to
determine if the subscription is renewed from today or from the day when his/her
account has expired.

Test plan:
Confirm that the behavior is correct before this patch and that it's
still the same after this patchset applied.

Signed-off-by: Aleisha Amohia <aleishaamohia@hotmail.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>