Bug 20942: Split debit and credit lines

Bug 20942: Split debit and credit lines

This patch splits the balance to match this object schema:

{
    balance             => #,
    outstanding_credits => {
        total => #,
        lines => [ credit_line_1, ..., credit_line_n ]
    },
    outstanding_debits  => {
        total => #,
        lines => [ debit_line_1, ..., debit_line_m ]
    }
}

This change is made to ease usage from the UI. Also because the
outstanding credits need to be applied to outstanding debits in order to
the balance value to make sense. So we still need to have each total.

Tests are added for this change, and the schema files are adjusted as
well.

To test:
- Apply this patch
- Run:
  $ kshell
 k$ prove t/db_dependent/api/v1/patrons_accounts.t
=> SUCCESS: Tests pass!
- Sign off :-D

staff_id is changed into user_id as voted on the dev meeting the RFC got
approved.

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>