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

.env.sample

@@ -1,4 +1,6 @@
-# Copy this file to .env, remove this comment and change the keys
+# Copy this file to .env, remove this comment and change the keys. For development
+# with docker this should mostly work out of the box other than setting the Slack
+# keys (for auth) and the SECRET_KEY.
 #
 # Please use `openssl rand -hex 32` to create SECRET_KEY
 
@@ -7,19 +9,28 @@ DATABASE_URL_TEST=postgres://user:pass@localhost:5432/outline-test
 SECRET_KEY=F0E5AD933D7F6FD8F4DBB3E038C501C052DC0593C686D21ACB30AE205D2F634B
 PORT=3000
 REDIS_URL=redis://redis:6379
-SLACK_KEY=71315967491.XXXXXXXXXX
-SLACK_SECRET=d2dc414f9953226bad0a356c794XXXXX
 URL=http://localhost:3000
 DEPLOYMENT=hosted
 ENABLE_UPDATES=true
-GOOGLE_ANALYTICS_ID=
 
+# Third party credentials (required)
+SLACK_KEY=71315967491.XXXXXXXXXX
+SLACK_SECRET=d2dc414f9953226bad0a356cXXXXYYYY
+
+# Third party credentials (optional)
+SLACK_VERIFICATION_TOKEN=PLxk6OlXXXXXVj3YYYY
+SLACK_APP_ID=A0XXXXXXX
+GOOGLE_ANALYTICS_ID=
+BUGSNAG_KEY=
+
+# AWS credentials (optional in dev)
 AWS_ACCESS_KEY_ID=notcheckedindev
 AWS_SECRET_ACCESS_KEY=notcheckedindev
 AWS_S3_UPLOAD_BUCKET_URL=http://s3:4569
 AWS_S3_UPLOAD_BUCKET_NAME=outline-dev
 AWS_S3_UPLOAD_MAX_SIZE=26214400
 
+# Emails configuration (optional)
 SMTP_HOST=
 SMTP_PORT=
 SMTP_USERNAME=