Double entry accounting milestone 5

[chongfun]

Turn Expense into an immutable posted financial source event.

After this milestone:

Expense
├── property
├── rentable_unit optional
├── expense_kind
├── amount_cents
├── paid_on
├── vendor_name
├── external_reference
├── description
└── lifecycle
      │
      ▼
JournalEntry
├── Dr mapped Expense account
└── Cr Cash

A reimbursement remains a separate event:

Expense
    │
    ├── Charge(reimbursement) -> Tenancy A
    └── Charge(reimbursement) -> Tenancy B

so:

$300 utility Expense
    Dr Utilities Expense     300
    Cr Cash                  300

$150 reimbursement Charge A
    Dr Tenant Receivable     150
    Cr Reimbursement Income  150

$150 reimbursement Charge B
    Dr Tenant Receivable     150
    Cr Reimbursement Income  150

produces exactly:

1 Expense
2 Charges
3 JournalEntries
6 Postings