Unfurling of Slack links (#487)

* First pass: Unfurling of Slack links

* Add authentication in db

* Call associate on Event correctly

* Add SLACK_APP_ID, remove SLACK_REDIRECT_URI, tidy env sample

* PR feedback

* Comment clarify

server/services/slack/index.js

@@ -0,0 +1,7 @@
+// @flow
+const Slack = {
+  id: 'slack',
+  name: 'Slack',
+};
+
+export default Slack;