monitoring: automatically deploy grafana notifiers (round 2)

Created by: bobheadxi

Why https://github.com/sourcegraph/sourcegraph/pull/11479 was reverted: https://github.com/sourcegraph/sourcegraph/pull/11479

This PR:

• updated conf check:
  • disables conf check when no alerts are configured
  • explicitly checks for sourcegraph/server
• still unable to work out where GRAFANA_PORT might be incorrectly set, so rename it to GRAFANA_INTERNAL_PORT (more descriptive anyway)

relevant changes here, this branch starts from a cherry-picked #11427 (already reviewed)

testing in:

• https://buildkite.com/sourcegraph/sourcegraph/builds?branch=docker-images-patch-notest%2Fgrafana
• https://buildkite.com/sourcegraph/sourcegraph/builds?branch=docker-images-patch-notest%2Ffrontend