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!

@maratkalibek @ayufan Any comments?

mentioned in issue infrastructure#1585 (closed)

assigned to @maratkalibek

added 4 commits

• da87b077...65c90492 - 3 commits from branch master
• 72222e58 - Improve CI alerts

Compare with previous version

merged