Update Stripe importer to rename tax field

This reflects the current CSV output from the "Export" button.

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.stripe: Generate payment entries for refunded payments.

In order to be refunded, the payment had to be paid in the first place, so
we should still generate an entry for it.