Simplify the various settings objects, to improve retrieval of 'parameters' from the base class

- Remove the GenericReferencedSettingsClass mixin
- Each subclass defines a very simple get_kwargs() method
- Now, at object level *and* class level we can perform lookup of settings and actually get proper data back
- Adds "model" option to setting (precursor of things to come)

InvenTree/templates/InvenTree/settings/setting.html

@@ -22,6 +22,9 @@
         {{ setting.description }}
     </td>
     <td>
+        {% if setting.model_name %}
+        <b>Model name: {{ setting.model_name }}</b>
+        {% endif %}
         {% if setting.is_bool %}
         <div class='form-check form-switch'>
             <input class='form-check-input boolean-setting' fieldname='{{ setting.key.upper }}' pk='{{ setting.pk }}' setting='{{ setting.key.upper }}' id='setting-value-{{ setting.key.upper }}' type='checkbox' {% if setting.as_bool %}checked=''{% endif %} {% if plugin %}plugin='{{ plugin.slug }}'{% endif %}{% if user_setting %}user='{{request.user.id}}'{% endif %}{% if notification_setting %}notification='{{request.user.id}}'{% endif %}>