Fix cost and totals for sponsored transactions.

[td202]

Purpose

Closes SPO-104

Update the cost/subtotal/total calculations for sponsored transactions.

Changes

Displayed cost:

• is the cost to the sponsor if the account is the sponsor account
• otherwise, is the cost to the sender if the account is the sender account (may be 0)
• otherwise, is absent.

Displayed subtotal:

• if the cost is present (i.e. account is sponsor or sender) and there is a (non-fee) change to the account's balance, this is the change
• otherwise, is absent.

Displayed total is the total balance change to the account.

Checklist

• My code follows the style of this project.
• The code compiles without warnings.
• I have performed a self-review of the changes.
• I have documented my code, in particular the intent of the
  hard-to-understand areas.
• (If necessary) I have updated the CHANGELOG.

[soerenbf]

Looks good 🙂