Double entry accounting milestone 6

[chongfun]

Implement:

Tenancy
└── SecurityDeposit
    └── SecurityDepositTransaction
        ├── received
        ├── refunded
        └── applied

with accounting:

Deposit received
  Dr Cash
  Cr Security Deposits Held

Deposit refunded
  Dr Security Deposits Held
  Cr Cash

Deposit applied
  Dr Security Deposits Held
  Cr Tenant Receivable

The last operation settles an already-existing Charge. It does not recognize income again.

At the end of the milestone:

• deposit requirement is visible on the Tenancy;
• deposit held is derived from accounting postings;
• money can be received into the deposit liability;
• held money can be refunded;
• held money can be applied against an existing Charge;
• liability can never go negative;
• deposits never become ordinary Receipts;
• security deposits never contribute to ordinary rent-received reporting.