Bug 10796: Add Koha::Patron::Category->effective_change_password method

Bug 10796: Add Koha::Patron::Category->effective_change_password method

This method checks whether the local $self->change_password is set to
override the OpacPasswordChange syspref (i.e. if it is set to a
bool) or undef, in which case it falls back to the value of the syspref

To test:
- Apply this patches
- Make sure the DB is updated:
  $ updatedatabase
- Update the schema files:
  $ dbic
- Run:
  $ kshell
 k$ prove t/db_dependent/Koha/Patron/Category.t
=> SUCCESS: Tests pass!
- Sign off :-D

Signed-off-by: Liz Rea <wizzyrea@gmail.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>