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

InvenTree/templates/InvenTree/settings/plugin_settings.html

@@ -101,6 +101,13 @@
                 <td>{% trans "This is a builtin plugin which cannot be disabled" %}</td>
             </tr>
             {% else %}
+            {% if plugin.is_sample %}
+            <tr>
+                <td><span class='fas fa-check-circle icon-blue'></span></td>
+                <td>{% trans "Sample" %}</td>
+                <td>{% trans "This is a sample plugin" %}</td>
+            </tr>
+            {% endif %}
             <tr>
                 <td><span class='fas fa-user'></span></td>
                 <td>{% trans "Commit Author" %}</td><td>{{ plugin.package.author }} - {{ plugin.package.mail }}{% include "clip.html" %}</td>

InvenTree/templates/InvenTree/settings/pricing.html

@@ -51,6 +51,13 @@
     </div>
     {% endif %}
 
+    <table class='table table-striped table-condensed'>
+        <tbody>
+            {% include "InvenTree/settings/setting.html" with key="CURRENCY_UPDATE_PLUGIN" icon="fa-cog" %}
+            {% include "InvenTree/settings/setting.html" with key="CURRENCY_UPDATE_INTERVAL" icon="fa-calendar-alt" %}
+        </tbody>
+    </table>
+
     <table class='table table-striped table-condensed' id='exchange-rate-table'></table>
 
 </div>