Moving `Account.amount` to `Account.credit` & `Account.debit`

[adamcharnock]

I've had a go at this this morning, and you can see the the core changes here.

Most of the lines changed are tests.

Notes:

• It is a straightforward but far-reaching change
• I was hoping this would drop out a bunch of complexity, but it did not
• my_leg.amount is still available as a property on the Django model. It is now always a positive value.
• Can still use the amount argument when creating a Leg, a deprication warning will be shown.
• I've added amount_legacy (I.e. debits are negative) to the LegView, so it is still available at my_leg.view.amount_legacy
• I'm not against merging this for Hordak 2.0 or 3.0
• The more time I spend with this branch the more it does feel like a much better way to structure things. I'm rather in favour of merging this.

Remaining Tasks

• [x] Test against MySQL
• [x] Trigger to ensure that only credit or debit is set
  • [x] Test
• [x] Trigger to ensure that credit/debit is > 0
  • [x] Test

Follow-up work

I'll do this next in another PR

• Ensure 'balance()' functionality hasn't changed (is the sign of the return value still the same?)
• Implement 'accounting_balance()' if necessary
• Test data migration (related: #111)

[adamcharnock]

I would love the input of both @nitsujri and @PetrDlouhy on this.

@nitsujri - I know this isn't precisely what you asked for, but what are your thoughts?

@PetrDlouhy - How comfortable would be feel about deploying this kind of change (once stable) into your production environment?

[adamcharnock]

All tests now passing 🎉🎉🎉