Merge branch 'ee-9258-support-alerts-from-external-prometheus-servers-frontend' into 'master'

Backport for gitlab-ee!9239

See merge request gitlab-org/gitlab-ce!24818

app/views/projects/services/prometheus/_show.html.haml

@@ -8,3 +8,5 @@
 
   .col-lg-9
     = render 'projects/services/prometheus/metrics', project: @project
+
+= render_if_exists 'projects/services/prometheus/external_alerts', project: @project