[ci.jenkins.io] Install customizable-header to allow providing system message banner everywhere

[dduportal]

Service(s)

ci.jenkins.io

Summary

When communicating about an outage on ci.jenkins.io, we want the administrative messages to be shown on all pages, not only on the homepage: #4490

As hinted in #4490 (comment), this is feature provided by the plugin customizable-header.

Let's install this plugin on ci.jenkins.io, to improve the communication to end users

Reproduction steps

No response

[dduportal]

Implemented in jenkins-infra/jenkins-infra#3819

Waaaaay better:

Thanks @mawinter69 @rantoniuk