accrual: Actually use RT caching as intended.

Basically none of the reports were reading or writing the RT link cache
because they didn't instantiate an rtutil.RT properly to do that.

accrual: Remove the consistency checker.

Everything it said was a problem has been done legitimately in our books at
one point or another.

* Variation in contract can happen in different line items of an invoice or
"group of contractor" situations.

* Variation in cost can happen because one invoice spans a period of time,
like donation matching programs. There is probably still value in a tool
that checks to make sure we use consistent rates each day, but that
affects all kinds of transactions, not just accruals, so it would be
done better in a separate tool.

* Variation in account happens because invoices legitimately span accrual
accounts, like donation matching programs with fees payable.

So: it's gone, good riddance.

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.

accrual: Make accruals consistent by entity on the accrual side.

It is more common than I realized that we split an invoice by
entity on the accrual side, so this supports that better.

It still disregards inconsistency between accrual entity and payment entity
for reporting purposes, to help keep reporting clean around automatic
imports.

The changes to BaseReport._report shook out because at this point, the group
key is effectively arbitrary and shouldn't be used for any reporting
purposes.

accrual: Load all books since args.since.

This lets the tool find and report accruals in the future.

accrual: Outgoing report doesn't put RT web links in < >.

The RT web interface doesn't handle angle brackets well, making the
`>` part of the link.

reports.accrual: Begin reporting.

This module basically reimplements the old payment-report+income-report,
in a single tool (after setting aside some of the checks that have moved
to the plugin). The aging report can be implemented here too when we need
that.

setup: Add overlooked PyYAML dependency.

Needed by meta_repo_links.

meta_entity: Adjust what entities are allowed based on today's books.

See the comments throughout for more discussion about what cases are
or aren't allowed, and why.