Send Slack notification on WaitForPackage failure

Resolves gitlab-com/gl-infra/delivery#1550 (closed).

Introduces slack-ruby-block-kit gem, which we've started using in ChatOps, and reorganizes the Slack-related code within the project a bit.

We may want a ~"technical debt" follow-up to:

1. Move other pieces of the code to use BlockKit
2. Re-use the new F_UPCOMING_RELEASE and RELEASE_MANAGERS constants => !1373 (merged)