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.

nbpy2017: Rename _RATE constants to _RBI.

This is a nonfunctional change that's just meant to better document
what these values actually represent.

nbpy2017: Handle complimentary tickets.

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

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.

util: Rename to strparse.

Better describes what the functions in the module do.

util: Add parse_currency_dec.

The current importers trim lots of extraneous symbols and whitespace from
currency strings before passing them to Decimal(). This function takes care
of all that in a single place.

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.