Slack step: allow multiple recipients

Imported from:

Description of the feature request

I would like to have my Slack notification go to multiple recipients (channels and/or users).

Can the channel fields be refactored to accept a list of notification targets? E.g. #build_results, $SLACK_LAST_COMMITER, @qa (assuming SLACK_LAST_COMMITER is an environment variable I exported in a previous step)?

The implementation could simply to loop over this collection and send a copy of the same message to each target.

Use case / for what or how I would use it

Send a message of a failing build to a general channel and to the committer who broke the pipeline.