Add balance breakdown to /accounting/ for each account.

See Issue #874.
Merge branch 'main' into issue874
Make accounting summary view @admin_required
Tweak accounting summary display

Transfers are grouped by incoming and outgoing, and the names for null
accounts are changed.
Fix silly bug in LineItemOptions.__unicode__

We were using %.2f to display a Decimal object, which instead needs to be
rendered as a string.
Merge pull request #1025 from learning-unlimited/issue874

Add balance breakdown to /accounting/ for each account.