Avoid triggering django.setup() when importing settings.

[koniiiik]

Importing the module drf_yasg.app_settings has a side effect of immediately triggering django.setup() once the call to getattr(settings, "LOGIN_URL", None) is encountered. This makes it impossible to import the default YASG settings at the beginning of your Django settings module in order to extend some of the default values with application-specific customizations.

This patch wraps the LOGIN_URL fallback into a lazy string object to delay the evaluation of Django settings.