Move settings validation out of startup.py

This also creates a new app common_initialization in order to provide a
place to run initialization code common to both the LMS and CMS.

lms/envs/common.py

@@ -2034,7 +2034,10 @@ INSTALLED_APPS = [
     'django.contrib.staticfiles',
     'djcelery',
 
-    # Initialization
+    # Common Initialization
+    'openedx.core.djangoapps.common_initialization.apps.CommonInitializationConfig',
+
+    # LMS-specific Initialization
     'lms_initialization.apps.LMSInitializationConfig',
 
     # Common views