Allow GitLab issue labels to point to a specific Slack Channel
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### Proposal
Add functionality to Slack integration allowing GitLab Issues labels to be the main driver of which Slack channel that issue will go to.
Why interested? Current Slack integration sent all GitLab issues into a single Slack channel as a "catch all" channel and requires human intervene to manually separate to teams or specific Slack channels. This will help reduce manual sifting and GitLab issues will surface in the correct Slack channels to the relevant teams when the team labels are applied.
**Customers requesting:**
1. https://gitlab.my.salesforce.com/0014M00001ldjLBQAY (internal only)
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