Bug 6810: Fix QA failures

Bug 6810: Fix QA failures

    - remove DateTime->now()
    - use Koha::DateUtils->dt_from_string;
    - use Pod2usage for the usage
    - use Modern::Perl
    - use branches table
    - Change letter code from MEMEXP to MEMBERSHIP_EXPIRY
    - review comments implemented
    - fix qa script comments

Bug 6810 - Fix QA failures

    - MembershipExpiryDaysNotice system preferences arragned alphabetical order.

Bug 6810 - Add sample notices

    - review comments implemented
    - default value of is_html field in letter table is 0

Signed-off-by: Alex Arnaud <alex.arnaud@biblibre.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>