Announce WordCamp deployments in Slack

This is a low priority, but it’d be helpful to announce wordcamp.org deployments in Slack, like we do with wordpress.org.

I added support for it to deployed.php in [dotorg13900], and tested the necessary changes to the deploy script in ~/deploy-wordcamp-tmp.sh on my sandbox.

Someone from Systems will need to commit/deploy those to the actual script, though.

Thanks πŸ™‚

#prio3