Utility for Slack

[rivernews]

Create a class so that we can easily post messages on Slack.

Let's use asyncio to try it out.

• [ ] Write async request class
  • [ ] Write integration test for request class: https://stackoverflow.com/questions/49936724/async-fixtures-with-pytest
• [ ] Post slack when 500
  • [ ] Write Slack service post using async request class