Issue lifecycle-bound silences

We should create/implement a tool/cron, that uses the GitLab and Alertmanager APIs to do the following:

• Scrape active silences in AM
• Look for a GitLab.com/ops link in the description
• If that issue behind that link is closed, remove the silence from AM

This enables us to have silences bound to the lifecycle of an issue/incident.

We should also consider posting an automated comment to the issue, that links to the silence.