zulip: reenable email auth

basic/overrides/stackspin-zulip-override.yaml

+---
+apiVersion: v1
+kind: ConfigMap
+metadata:
+  namespace: stackspin-apps
+  name: stackspin-nextcloud-override
+data:
+  values.yaml: |
+    zulip:
+      environment:
+        SETTING_AUTHENTICATION_BACKENDS: '("zproject.backends.GenericOpenIdConnectBackend","EmailAuthBackend")'