Fixed #36138 -- Changed ADMINS and MANAGERS settings to lists of strings.

Previously, the ADMINS and MANAGERS settings were lists of (name, address)
tuples (where the name had been unused). Deprecated use of tuples.

Updated settings value sanity checks, and changed from ValueError to
ImproperlyConfigured.

tests/middleware/tests.py

@@ -389,7 +389,7 @@ class CommonMiddlewareTest(SimpleTestCase):
 
 @override_settings(
     IGNORABLE_404_URLS=[re.compile(r"foo")],
-    MANAGERS=[("PHD", "PHB@dilbert.com")],
+    MANAGERS=["manager@example.com"],
 )
 class BrokenLinkEmailsMiddlewareTest(SimpleTestCase):
     rf = RequestFactory()