chore: Remove env variables in webpack bundle (#1353)

* chore: Remove env variables in webpack bundle

* remove unused globals

* refactor: consolidate window.env calls to single file

* fix: Slack client side integration auth

* fix: developers url

app/utils/domains.js

@@ -0,0 +1,7 @@
+// @flow
+import { stripSubdomain } from "shared/utils/domains";
+import env from "env";
+
+export function getCookieDomain(domain: string) {
+  return env.SUBDOMAINS_ENABLED ? stripSubdomain(domain) : domain;
+}