fix: Add server-side proxy support via fetch-with-proxy (#2044)

* fix: Add server-side proxy support via fetch-with-proxy

closes #1893

For some fun discussion on why this is required, see this issue: https://github.com/nodejs/node/issues/8381

* lint

server/slack.js

@@ -1,6 +1,6 @@
 // @flow
 import querystring from "querystring";
-import fetch from "isomorphic-fetch";
+import fetch from "fetch-with-proxy";
 import { InvalidRequestError } from "./errors";
 
 const SLACK_API_URL = "https://slack.com/api";