chore: Upgrade Babel, Jest, Eslint (#1437)

* chore: Upgrade Prettier 1.8 -> 2.0

* chore: Upgrade Babel 6 -> 7

* chore: Upgrade eslint plugins

* chore: Add eslint import/order rules

* chore: Update flow-typed deps

server/slack.js

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