Bug 6810: Add new systempreference (MembershipExpiryDaysNotice)

Bug 6810: Add new systempreference (MembershipExpiryDaysNotice)

Signed-off-by: Indranil Das Gupta (L2C2 Technologies) <indradg@gmail.com>
Bug 6810 - Fix QA failures

- Use KohaDates to convert dateexpiry
- remove MYSQL specifics methods for date handling in
  GetUpcomingMembershipExpires
- make the script membership_expiry.pl write in Koha system logs
- add tests

Signed-off-by: Indranil Das Gupta (L2C2 Technologies) <indradg@gmail.com>
Bug 6810 - Fix QA failures:

  - use Koha::DateUtils instead of Koha::Template::Plugin::KohaDates,
  - Add test with syspref MembershipExpiryDaysNotice equals 0 and undef,
  - fix (new) test failure (when MembershipExpiryDaysNotice is undef).

Signed-off-by: Indranil Das Gupta (L2C2 Technologies) <indradg@gmail.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>