Bug 16853: Move changepassword to Koha::Patron->update_password

Bug 16853: Move changepassword to Koha::Patron->update_password

This patch moves the code from C4::Members::changepassword to
Koha::Patron->update_password

Test plan:
Change your password at the OPAC and the staff interface
This should work as before

Signed-off-by: Marc Véron <veron@veron.ch>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
I rebased this on top of 16849 because they were conflicting.
Tests pass, code looks good (as usual) and I checked both OPAC
and staff password change work as expected.

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>