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

package.json

@@ -169,6 +169,7 @@
     "slate-edit-list": "^0.10.1",
     "slate-md-serializer": "^1.0.6",
     "slate-paste-linkify": "^0.5.0",
+    "slate-plain-serializer": "^0.4.15",
     "slate-prism": "^0.4.0",
     "slate-react": "^0.10.19",
     "slate-trailing-block": "^0.4.0",