management: Refactor checkconfig code to live in library.

This makes it possible to call this from other management commands.

zerver/management/commands/checkconfig.py

@@ -5,16 +5,10 @@ from typing import Any
 from django.conf import settings
 from django.core.management.base import BaseCommand
 
+from zerver.lib.management import check_config
+
 class Command(BaseCommand):
     help = """Checks your Zulip Voyager Django configuration for issues."""
 
     def handle(self, *args: Any, **options: Any) -> None:
-        for (setting_name, default) in settings.REQUIRED_SETTINGS:
-            try:
-                if settings.__getattr__(setting_name) != default:
-                    continue
-            except AttributeError:
-                pass
-
-            print("Error: You must set %s in /etc/zulip/settings.py." % (setting_name,))
-            sys.exit(1)
+        check_config()