Bug 21727: Add ->adjust to Koha::Account::Line

Bug 21727: Add ->adjust to Koha::Account::Line

This patch introduces a high-level method for updating existing debits,
similar to Koha::Account::add_credit and Koha::Account::add_debit.

It updates the Koha::Account::Line object, and adds an offset. It also
logs if required.

It whitelists the account types that are allowed to be adjusted, just in
case some others require adjusting, having a single source for truth.

To test:
- Apply this patches
- Run:
  $ kshell
 k$ prove t/db_dependent/Koha/Account/Lines.t
=> SUCCESS: Tests pass!

Sponsored-by: PTFS Europe
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>