Specify order currency (#4698)

* Add 'order_currency' to the various external order models

- By default will use the currency specified for the supplier (or customer)
- Can be specified per order, also

* Display order currency on order pgae

* Add 'order_currency' field

* Enable "blank" currency option (to default to the currency specified by the referenced company

* Fix default currency code when adding line items

* Remove 'total_price_currency' serializer field

- Now replaced with 'order_currency' for greater flexibility

* Bump api_version.py

* Update default order report templates

* Updated docs

* More docs updaes

* Adjust unit tests

* Use 'order_currency' in order tables

* Update purchase order api unit tests

docs/README.md

@@ -21,7 +21,7 @@ $ pip install -r requirements.txt
 To serve the pages locally, run the following command (from the top-level project directory):
 
 ```
-$ mkdocs serve -a localhost:8080
+$ mkdocs serve -f docs/mkdocs.yml -a localhost:8080
 ```
 
 ## Edit Documentation Files