budget/ledger unavailable shouldn't include overEncumbered amounts

Overview

This wasn't clear until working through some examples with dennisbridges but when creating encumbrances, the corresponding budget/ledger's unavailable amount should not include the amount overEncumbered portion of the encumbrance.

Example:

• We have the following budget:
  • allocated: $100
  • available: $100
  • unavailable: $0
  • encumbered: $0
  • awaitingPayment: $0
  • expended: $0
  • allowableEncumbrance: 110% (remaining allowable encumbrance: $110)
• An order is placed for $110
  • allocated: $100
  • available: $0
  • unavailable: $100
  • encumbered: $110
  • awaitingPayment: $0
  • expended: $0
  • overEncumbered: $10
  • allowableEncumbrance: 110% (remaining allowable encumbrance: $0)

In general the available and unavailable numbers only apply to "real" money, not overEncumbered/overExpended.

Acceptance Criteria

• the unavailable calculation in budget/ledger upon encumbrance is updated as described above
• unit tests are updated

NOTE: If the story is being handled before MODFISTO-59, just handle POST, else handle both POST and PUT operations