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/startup.py

@@ -18,8 +18,6 @@ from openedx.core.djangoapps.monkey_patch import django_db_models_options
 import xmodule.x_module
 import lms_xblock.runtime
 
-from startup_configurations.validate_config import validate_lms_config
-
 log = logging.getLogger(__name__)
 
 
@@ -45,9 +43,6 @@ def run():
     xmodule.x_module.descriptor_global_handler_url = lms_xblock.runtime.handler_url
     xmodule.x_module.descriptor_global_local_resource_url = lms_xblock.runtime.local_resource_url
 
-    # validate configurations on startup
-    validate_lms_config(settings)
-
 
 def add_mimetypes():
     """