Token auth (#434)

* Embed device platform information into token request

* Remove username and password from userProfile

* Display icon to show if profile has associated user token

* Remove username / password from login settings screen

* Refactor login procedure around token auth

* Refactoring

* Add profile login screen

- Username / password values are not stored
- Just to fetch api token

* Login with basic auth

* Pass profile to API when connecting

* Remove _BASE_URL accessor

- Fixes URL caching bug

* Add more context to login screen

* Add helper functions for unit tests

- Change default port to 8000 (makes testing easier with local inventree instance)

* api.dart handles basic auth now

* fix api_test.dart

* Further test improvements

* linting fixes

* Provide feedback when login fails

* More linting

* Record user details on login, and display in "about" widget

* Fix string lookup

* Add extra debug

* Fix auth values

* Fix user profile test

lib/inventree/purchase_order.dart

@@ -131,7 +131,17 @@ class InvenTreePurchaseOrder extends InvenTreeModel {
     }
   }
 
-  String get totalPriceCurrency => getString("total_price_currency");
+  // Return the currency for this order
+  // Note that the nomenclature in the API changed at some point
+  String get totalPriceCurrency {
+    if (jsondata.containsKey("order_currency")) {
+      return getString("order_currency");
+    } else if (jsondata.containsKey("total_price_currency")) {
+      return getString("total_price_currency");
+    } else {
+      return "";
+    }
+  }
 
   Future<List<InvenTreePOLineItem>> getLineItems() async {