Investigate if embargo/lockdown can be better communicated

[richardlau]

Currently, when we lock down the CI for security release preparation we add a "System message" explaining that access is restricted:

build/doc/jenkins-guide.md

Lines 101 to 106 in de38bc6

	Add a Jenkins "system message" in https://ci.nodejs.org/configure. Something like:
	```html
	<h1 style="color:red">system is under embargo for a security release</h1>
	<h2>For solidarity, even if you have access, please don't start unrelated jobs</h2>
	<p>Refs: <a href="https://github.com/nodejs/build/issues/xxx">https://github.com/nodejs/build/issues/xxx</a></p>
	```

Unfortunately:

• The message only appears on https://ci.nodejs.org/, not on any of the sub pages (e.g. https://ci.nodejs.org/job/node-test-pull-request/).
• If you do not have access, there is no indication at all, e.g. from Access Denied jenkins-alerts#4575:
  

I did come across https://plugins.jenkins.io/customizable-header/, which may be worth investigating to improve the user experience during a CI lockdown. If it would work, it would mean adding a new plugin to the public CI.

[richardlau]

With the plugin installed I now see this (i.e. we get a message on the sub pages):

I'll try to confirm with someone that doesn't have access if they also see the message.

[richardlau]

Got confirmation from a couple of people who don't have access to the CI that they're also seeing the system message, so I think that's a win.

I'll need to update the process documentation -- we don't need to set the "System message" any more if we use the plugin as otherwise we get a repeated message on the top level https://ci.nodejs.org/.