Add bot command for issue validation

@jimender2 has been helping validate old issues and currently pinging me on issues

We considered pinging @gitlab-org/coaches instead so I don't need to pick them all up but that will flood 40 inboxes/todos regardless of OOO etc

We also considered using the new contributor platform but there's no easy way for users to know how to trigger a specific URL

We lastly considered adding labels which could then be used to generate a daily triage report, but this seems a bit slow and cumbersome.

The simplest approach seems to be creating a new slack channel (we don't want to flood the #mr-coaches channel), and have etiquette around reactions 👀 when looking ✅ once responded/completed

We could always follow up with label/triage report at a later stage if we find issues slipping through the cracks

/cc @zillemarco @gitlab-org/developer-relations/contributor-success