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/models/User.js

@@ -2,12 +2,11 @@
 import crypto from 'crypto';
 import bcrypt from 'bcrypt';
 import uuid from 'uuid';
+import JWT from 'jsonwebtoken';
 import { DataTypes, sequelize, encryptedFields } from '../sequelize';
 import { uploadToS3FromUrl } from '../utils/s3';
 import mailer from '../mailer';
 
-import JWT from 'jsonwebtoken';
-
 const BCRYPT_COST = process.env.NODE_ENV !== 'production' ? 4 : 12;
 
 const User = sequelize.define(