Improve CI alerts

This MR moves all CI related alerts to one place and improve some of the existing alerts.

Notice: Alerts send to channel="ci" are currently not handled properly because our #ci slack channel was recently renamed to #ci-cd. Related fix for alertmanager configuration is here - https://gitlab.com/gitlab-cookbooks/gitlab-prometheus/merge_requests/220.

We should also decide if we leave ci here and only change target channel in the receiver, or should we also update ci to ci-cd in rules files and howto/alerts_manual.md.

Requires gitlab-cookbooks/gitlab-prometheus!220 (merged) to be merged first!