[Feature] Add Slack integration

[andrewjkerr]

Here's a wishlist for Slack integration:

• A webhook to Slack that will post into a "mentor feed" channel on ticket updates.
• A route that users in Slack can use via a slash command to create, cancel, and get their tickets.

[andrewjkerr]

I've been working on this for Swamphacks: master...andrewjkerr:master

I'm going to do a once-over and see if there are any optimizations to be made (or dumb mistakes I made) before opening a PR. Comments are welcome!

Also, thank you very much for creating this! It saved me quite a bit of work re-working a former Slackbot I built. 😄