Create accounting-oriented database view on Legs table

[adamcharnock]

As per the discussion in #44 (comment, comment).

This will add the columns:

• debit - the debit amount, always positive, or null if the leg is a credit
• credit - the credit amount, always positive, or null if the leg is a debit
• amount - the amount, always positive
• type - DR or CR

[adamcharnock]

I've just realised that the implementation here only works for leaf accounts. Either:

1. Make it work with non-leaf accounts
2. Show NULL balance for non-leaf accounts

[adamcharnock]

Relatedly, we now also have #126, which provides a db-level function for calculating account balances.

[adamcharnock]

Show NULL balance for non-leaf accounts

This is what I implemented in the end. I figured this would give the biggest bang for complexity/performance buck.