Double entry accounting milestone 7

[chongfun]

Replace:

PaymentDocument
└── PaymentIngestion
    └── Receipt

with:

SourceDocument
└── ImportedTransaction
    └── confirmed_source
        ├── Receipt
        └── SecurityDepositTransaction(received)

The final confirmation semantics are:

tenant_receipt
    -> Receipt
    -> Dr Cash
       Cr Tenant Receivable

security_deposit
    -> SecurityDepositTransaction(received)
    -> Dr Cash
       Cr Security Deposits Held

An imported transaction itself never posts anything.