Slack notifications for recovered pipelines
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### Description
Slack notification options for pipeline currently include all pipeline jobs regardless of status and only failed jobs. This means that slack channels either get a lot of spam about jobs that worked fine mixed in with failures, or only failures with no indication that they've been resolved.
### Proposal
The proposal is to add a new checkbox to the Slack integration configuration allowing notification of a channel when a pipeline has recovered from a failure state.
### Links / references
* gitlab-ce#24644 - Similar feature request for emails
issue