patreon: Exclude tax from full amount.

This makes the balancing logic in entry generation work the way we want.

patreon: Update importer for this month's change.

Literally all they did was change the title of the amount column
(maybe it doesn't assume USD anymore?).

patreon: Update importer for recent patron changes. RT#12747

All they did was change a couple of column names.

github: Update to match fiscal host reporting.

In general the fiscal host reporting isn't as nice as the project reporting,
so I hope someday we can revert this change, but until then that's what
we're using.

nbpy2017: Report item rate names and final unit prices.

This keeps more of the RBI calculation in the templates, where it belongs.
Template logic has become more capable since this importer was first
written, which makes this change practical.

csv: Support importing squared CSV spreadsheets.

See the test comment for more rationale.

eventbrite: Sales report can include currency.

It's a selectable column that I apparently failed to include in my last
report. Keep USD around as a default for convenience.

benevity: Version the current importer.

The report format has changed slightly in 2019.

nbpy2017: Handle complimentary tickets.

The conference is offering these to people who proposed talks this year.

brightfunds: New importer.

This commit adds infrastructure to treat XLS files like CSV files, and
import them using the base classes that already exist for that.

hooks.ledger_entry: Look up templates dynamically.

If there's a 'ledger entry' key in the entry data, use that value as the
name of the template to load. Thanks to this, nbpy2017 could collapse
multiple importers into one.

Otherwise, build a default template name based on the importer source, and
try to use that.

All the configuration names now end with "ledger entry" instead of starting
with "template". This makes it clearer what they're for, in case we
support other kinds of output templates in the future.

I ended up changing the names of some of the importers so the default
template name was nice, rather than specifying template names for all of
them, to reduce the amount of name discrepancies across the codebase.

importers.nbpy2017: Yield entries for the original invoice.

This lets us write up a cleaner separation between the original act of
invoicing and the Stripe payment.

importers.nbpy2017: Don't deal with refunds.

Only one refund was actually issued, so we can deal with that manually. But
the code did its job of revealing those cases and checking we were handling
them well.

importers: New importer for North Bay Python invoices.

This probably has no use outside Conservancy.

import2ledger: First version.

This is a pretty feature-complete version 1.
I don't know why I waited this long to commit anything.