Add custom logic for WATO Cash FI

[ansonjwhe]

Regular funding item:

• funding allocation: set on FI creation, should not change after creation
• funding spent: dynamically aggregated as sum of cost of purchases linked to FI

WATO Cash funding item:

• funding allocation: should serve as a live balance for WATO
• funding spent: unused field
• when purchase transitioned to spent that is linked to WATO cash: should decrease funding allocation by same amount