Add Rollback command with `check` subcommand

Robert Speicherrequested to merge
rs-rollback into master

Proof of concept for gitlab-com/gl-infra/delivery#1512 (closed)

There's a possibility in the future that we'll want to determine in release-tools if a deployment is safe to roll back, so it may make sense to move most of the functionality here into release-tools, and simply trigger a pipeline there from ChatOps as we do for some other commands.

But as a first step, I did this entirely in ChatOps for now.

Examples:

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