Update demo data hook to copy media files (#3441)

* Simplify settings.py / config.py

- get_setting function has been streamlined
- move some functions into config.py

* Spelling fix: IGNORRED is IGNORED

* Ensure yaml is installed as part of docker image

- invoke path is still mucking us around

* Fix broken migration

* Copy media files from demo dataset when installing test data

* Cleanup settings.py

* Fix for configuration file traversal

* Line fix

* Update quickstart guide for docker

* Allow plugin file and plugin dir to be specified in configuration file

* Cleanup config template file

* Allow secret_key information to be provided in configuration file

* Adjust root paths for CI tests

* resolve paths

* Revert paths for CI step

* remove dead code

* Revert configuration variables to old names

- Prevent breaking changes

* Simplify secret key generation

* Fix default timeout for background worker process

* Revert change for customization options

InvenTree/plugin/registry.py

@@ -22,6 +22,8 @@ from django.utils.text import slugify
 from maintenance_mode.core import (get_maintenance_mode, maintenance_mode_on,
                                    set_maintenance_mode)
 
+from InvenTree.config import get_setting
+
 from .helpers import (IntegrationPluginError, get_plugins, handle_error,
                       log_error)
 from .plugin import InvenTreePlugin
@@ -199,7 +201,7 @@ class PluginsRegistry:
         if settings.TESTING:
             custom_dirs = os.getenv('INVENTREE_PLUGIN_TEST_DIR', None)
         else:
-            custom_dirs = os.getenv('INVENTREE_PLUGIN_DIR', None)
+            custom_dirs = get_setting('INVENTREE_PLUGIN_DIR', 'plugin_dir')
 
             # Load from user specified directories (unless in testing mode)
             dirs.append('plugins')