caddy-frontend: Introduce Master Introspection Frontend

This is a frontend used to provide information about various aspect of the
master instance status, like rejected slaves, failed failsafe slave
publication and maybe more.
10 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
passed Cloudooo.UnitTest-Master

00:16:48

passed ERP5.StandaloneDeploymentScriptTest-Debian.Buster

00:27:33

passed ERP5.StandaloneDeploymentScriptTest-Debian.Buster.Master

08:03:52

passed SlapOS-Master.StandaloneDeploymentScriptTest-Debian.Buster

00:35:06

passed SlapOS.Eggs.UnitTest-Master.Python2

00:14:47

passed SlapOS.Eggs.UnitTest-Master.Python3

00:14:53

passed SlapOS.KVM.ResilienceTest-Master

00:20:08

passed SlapOS.SlapRunner.ResilienceTest-Master

00:24:15

failed SlapOS.SlapRunner.ResilienceTest-Master.ERP5

01:31:35

passed Wendelin.StandaloneDeploymentScriptTest-Debian.Buster

00:34:10