Implement request verification

Slack documentation on how to verify requests: https://api.slack.com/docs/verifying-requests-from-slack
For HMAC: https://github.com/briansmith/ring https://briansmith.org/rustdoc/ring/hmac/index.html

It would be nice if it can be implemented as a guard:

https://github.com/SergioBenitez/Rocket/issues/775
https://api.rocket.rs/v0.4/rocket/data/trait.FromData.html