Adds API endpoints for viewing and updating plugin settings

A lot of code updates / refactoring here to get this to work as expected

InvenTree/plugin/plugin.py

@@ -1,7 +1,9 @@
 # -*- coding: utf-8 -*-
-"""Base Class for InvenTree plugins"""
-
+"""
+Base Class for InvenTree plugins
+"""
 
+from django.db.utils import OperationalError, ProgrammingError
 from django.utils.text import slugify
 
 
@@ -10,6 +12,9 @@ class InvenTreePlugin():
     Base class for a plugin
     """
 
+    def __init__(self):
+        pass
+
     # Override the plugin name for each concrete plugin instance
     PLUGIN_NAME = ''
 
@@ -36,5 +41,22 @@ class InvenTreePlugin():
 
         return self.PLUGIN_TITLE
 
-    def __init__(self):
-        pass
+    def plugin_config(self, raise_error=False):
+        """
+        Return the PluginConfig object associated with this plugin
+        """
+
+        try:
+            import plugin.models
+
+            cfg, _ = plugin.models.PluginConfig.objects.get_or_create(
+                key=self.plugin_slug(),
+                name=self.plugin_name(),
+            )
+        except (OperationalError, ProgrammingError) as error:
+            cfg = None
+
+            if raise_error:
+                raise error
+
+        return cfg