Exchange rate plugin (#5667)

* Add plugin mixin class for supporting exchange rates

* Split some mixin classes out into their own files

- mixins.py is becoming quite bloated!

* Add some new settings for controlling currency updates

* Adds basic plugin implementation

* Refactor existing implementation

- Builtin plugin uses frankfurter.app API
- Better error / edge case handlign

* Add sample plugin for currency exchange

* Allow user to select which plugin to use for plugin updates

* Observe user-configured setting for how often exchange rates are updated

* Updates for some of the sample plugins

* Fix plugin slug

* Add doc page

* Document simple example

* Improve sample

* Add blank page for currency settings info

* More info in "config" page

* Update docs again

* Updated unit tests

* Fill out default settings values when InvenTree runs

* Add log messages

* Significant improvement in default settings speed

- Use bulk create
- Be efficient
- Dont' be inefficient

* More strict checks

* Refactor default values implementation

- Don't run at startup
- Run on list API
- Implement generic @classmethod

docs/docs/settings/currency.md

@@ -0,0 +1,31 @@
+---
+title: Currency Support
+---
+
+## Currency Support
+
+InvenTree provides support for multiple currencies, allowing pricing information to be stored with base currency rates.
+
+### Configuration
+
+To specify which currencies are supported, refer to the [currency configuration](../start/config.md#supported-currencies) section
+
+### Currency Conversion
+
+Currency conversion is provided via the [django-money](https://github.com/django-money/django-money) library. Pricing data can be converted seamlessly between the available currencies.
+
+### Currency Rate Updates
+
+Currency conversion rates are periodically updated, via an external currency exchange server. Out of the box, InvenTree uses the [frankfurter.app](https://www.frankfurter.app/) service, which is an open source currency API made freely available.
+
+#### Custom Rate Updates
+
+If a different currency exchange backend is needed, or a custom implementation is desired, the currency exchange framework can be extended [via plugins](../extend/plugins/currency.md). Plugins which implement custom currency exchange frameworks can be easily integrated into the InvenTree framework.
+
+### Currency Settings
+
+In the [settings screen](./global.md), under the *Pricing* section, the following currency settings are available:
+
+{% with id="currency-settings", url="settings/currency.png", description="Currency Exchange Settings" %}
+{% include 'img.html' %}
+{% endwith %}