accrual: AccrualPostings.make_consistent() groups accruals by date.

This accommodates cases of contracts without separate invoices,
where a series of payments are scheduled over time.

The dance we used to do of group-by-invoice, then make consistent was
already suspect. It was originally motivated by the consistency checks that
are now gone. Use this opportunity to clean up and just make make_consistent
a classmethod.

accrual: Add columns to the aging report. RT#11439.

This adds almost all the metadata that's relevant to accruals.
I considered adding statement, but that cuased rows to get spaced out a lot,
and statement's kind of a low-value column, so I decided against it.

Ultimately I would like to make this configurable but that's for the
future.

accrual: Aging report uses group since last nonzero. RT#11600.

This makes it consistent with our other reports. The fact that it wasn't
already was basically an oversight.

accrual: Outgoing report uses first rt-id as primary ticket number.

This makes it consistent with the reporting and our general rule
that the first link is primary.

accruals: AccrualPostings only reports inconsistent cost per currency.

Of course if an accrual has multiple currencies, it'll probably have
different costs, and that's fine.

accrual: Filter opening balance txn before main reporting.

So far we've been implicitly relying on this by the user passing search
terms that filter out the opening balance transaction. That will stop
happening with the aging report, so we need to do it ourselves.

tests: Add test for trying to make outgoing report without rt-id.

Changes to other test cases are to make them *not* trip up this
error after adding it to the books.

reports.accrual: Exclude payments from default output. RT#11294.

This makes the output more useful for broad searches like on an
entity. Invoices that cross FY boundaries will appear to be paid
without being accrued, and so would appear when we were just
filtering zeroed-out invoices.

If we integrate the aging report into this module in the future,
that'll need to follow different logic, and just filter out
zeroed-out invoices. But the basic balance report and outgoing
report are more workaday tools, where more filtering makes them
more useful.