Add Slack Intergration

[prkumar]

This Issue will track progress for creating a Slack Webhook for Emissary.

We have an app up on Slack, to which any member of the Anchorbuddies Slack team should have access. To distribute the app, we need an OAuth Redirect URL: follow this guide to handle authorization.

Further, you can find the previous team's Slack implementation at routes/checkIn.js