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/apps.py

@@ -4,17 +4,17 @@ from __future__ import unicode_literals
 from django.apps import AppConfig
 from maintenance_mode.core import set_maintenance_mode
 
-from plugin.registry import plugins
+from plugin import plugin_registry
 
 
 class PluginAppConfig(AppConfig):
     name = 'plugin'
 
     def ready(self):
-        if not plugins.is_loading:
+        if not plugin_registry.is_loading:
             # this is the first startup
-            plugins.collect_plugins()
-            plugins.load_plugins()
+            plugin_registry.collect_plugins()
+            plugin_registry.load_plugins()
 
             # drop out of maintenance
             # makes sure we did not have an error in reloading and maintenance is still active