* 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
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