Support multiple Slack integration for the same repository
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### Problem to solve
Our [Slack Integration](https://docs.gitlab.com/ee/user/project/integrations/gitlab_slack_application.html) allows users to send messages to multiple slack channels however we cannot have multiple exclusive notification settings for each channel.
### Proposal
We should allow users to have exclusive notification settings for each channel. This means that they can have different triggers for each channel that they want to add. For example, users should be able to achieve the following.
> - slack channel A - Messages are send from all MR created from repository(A merge request is created, merged, closed, or reopened), MR comments are also included(A comment is added)
> - slack channel B - the new one - Messages are send each time a push is added to repository on `hotfix/*` branch.
### Intended users
* [Parker (Product Manager)](https://about.gitlab.com/handbook/product/personas/#parker-product-manager)
* [Delaney (Development Team Lead)](https://about.gitlab.com/handbook/product/personas/#delaney-development-team-lead)
* [Sasha (Software Developer)](https://about.gitlab.com/handbook/product/personas/#sasha-software-developer)
* [Alex (Security Operations Engineer)](https://about.gitlab.com/handbook/product/personas/#alex-security-operations-engineer)
* [Cameron (Compliance Manager)](https://about.gitlab.com/handbook/product/personas/#cameron-compliance-manager)
### Feature Usage Metrics
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### Does this feature require an audit event?
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