add system check for SENDGRID_API_KEY setting

mails/apps.py

@@ -6,3 +6,4 @@ class MailsConfig(AppConfig):
 
     def ready(self):
         from . import signals  # noqa
+        from . import checks  # noqa

mails/checks.py

+"""Mails app system checks."""
+
+from django.conf import settings
+from django.core.checks import Warning, register
+
+
+@register()
+def check_sendgrid_api_key_is_set(app_configs, **kwargs):
+    errors = []
+
+    if getattr(settings, 'SENDGRID_API_KEY', None) is None:
+        errors.append(
+            Warning(
+                'SENDGRID_API_KEY is not set, sending emails will fail',
+                obj=settings,
+                id='mails.W001',
+            )
+        )
+
+    return errors

mails/settings.py

 """Mails app settings."""
 from django.conf import settings
-import warnings
 
 NOTIFICATIONS_ADDRESS = getattr(settings, 'MAILS_NOTIFICATIONS_ADDRESS')
 ENABLED = getattr(settings, 'MAILS_ENABLED')
 RAISE_EXCEPTIONS = getattr(settings, 'MAILS_RAISE_EXCEPTIONS')
-
-if getattr(settings, 'SENDGRID_API_KEY', None) is None:
-    warnings.warn('SENDGRID_API_KEY is not set, sending emails will fail')