Settings: Define new configuration for "lock total" value

Purpose:

Some libraries want to be able to define the default value of the Invoices "lock total". This will ideally be configurable via a Setting.

Refers to Acquisitions SIG Implementer Topic 32

User story statement(s):

As a system administrator,
I want to define the default value for the "lock total" field
so that users will follow institutionally-defined workflows

Scenarios:

1. Add new sub heading to settings: UINV-450
   • Given user navigates to settings>invoices
   • When sub heading are displayed
   • Then list includes a heading for invoice defaults
2. Scenario:
   • Given that an invoice defaults subheading has been implemented 
   • When a user opens Invoice Defaults 
   • Then the option to set a default value for "lock total" is available 
   • When the value is changed, the user may click "save" to apply the new setting.