Update to post-then-commit flow for purchases

Just like return invoices currently do in the 2-2-dev branch.

Create an Avatax "SalesOrder" to capture tax info prior to the "confirm" step.
At the "confirm" step create an uncommitted Avatax "SalesInvoice" to capture tax information.
Then after the order is complete commit the existing SalesInvoice.

Also:

- Provide a DB migration to create uncommitted SalesInvoice records for existing orders in the "confirm" state
- Proivde a rake task to backfill SalesInvoices for historical orders that have been sent to Avatax
- Remove now-unused "avalara" gem
- Remove obsolete "endpoint" config
yah i'd love to figure out how to make sure included tax works correctly, i am a bit hesitant to do so w/o sitting down and really figuring out how it's supposed to work though. on the readme we do have a warning for now that it doesn't work with included taxes: '"Additional tax" (e.g. US taxes) is supported but "included tax" (e.g. VAT) is not' (https://github.com/bonobos/spree_avatax/blob/d7f3245/README.md#known-issues)