feat: Enforce single team when self-hosted (#1954)

* fix: Enforce single team when self hosting

* test: positive case

* refactor

* fix: Visible error message on login screen for max teams scenario

* Update Notices.js

* lint

server/utils/authentication.js

@@ -0,0 +1,7 @@
+// @flow
+
+export function getAllowedDomains(): string[] {
+  // GOOGLE_ALLOWED_DOMAINS included here for backwards compatability
+  const env = process.env.ALLOWED_DOMAINS || process.env.GOOGLE_ALLOWED_DOMAINS;
+  return env ? env.split(",") : [];
+}