Support loan borrowing, repayment, and interest

[curtailment]

I believe we should add transaction types for borrowing assets (i.e. taking out a loan) and associated events for repayment and interest accrued (payable) on the loan.

There are many scenarios that this would be useful:

• Aave, a popular DeFi app, allows you to deposit assets to earn interest plus use them as collateral to borrow other assets.
• Also in DeFi, minting Dai against ETH with Maker, or minting sUSD against SNX with Synthetix.
• Taking a collateralised loan from a BlockFi/Nexo/Celsius.
• Margin/leverage trading could also be recorded in BittyTax if loans are supported (otherwise balances go negative since your purchasing power is greater than the amount you deposited).

I may suggest naming the events as:

• Loan-Received not a taxable event, similar to Gift-Received
• Loan-Repaid I don't think this would be taxable either, may be implemented similarly to Gift-Spouse
• Loan-Interest or Interest-Payable to tell the accounting system that the outstanding loan amount has increased.

[nanonano]

The margin-trading branch now includes the new transaction types below, see https://github.com/BittyTax/BittyTax/commit/a19e44d6812f2b18e7e8f60e63394fc1ce7ac6a5.

1. Loan - amount you have borrowed as a loan
2. Loan-Repayment - the repayment in part of full of the borrowed amount
3. Loan-Interest - any interest payments on the amount you have borrowed

Both Loan-Repayment and Loan-Interest result in a disposal if payment is in crypto.

Looking for beta testers :)

[nanonano]

Closing this issue now that the margin trading branch has been merged into master.