Package AlertManager to begin providing proactive notifications

With the growing number of Prometheus metrics we support, we need to start offering a way to generate alerts proactively when GitLab may need attention.

The easiest and smallest way to do this, is to package the Prometheus AlertManager, which can process and handle configured alerts. We can expose some simple configuration to define SMTP settings to generate email alerts for now, and expand to additional destinations later.

Once we have this, additional alerts can be created by individual teams.