The test system I set up to verify resolution of customer(s)' issues driving this change, required
additional changes in order to fully resolve the issues at hand. As a result I am bundling these
changes:

- Moves nodes to being store in separate config files outside of the main config file (improves performance) [FIXED JENKINS-27562]
- Makes the Jenkins is loading screen not block on the extensions loading lock [FIXED JENKINS-27563]
- Removes race condition rendering the list of executors [FIXED JENKINS-27564] [FIXED JENKINS-15355]
- Tidy up the locks that were causing deadlocks with the once retention strategy in durable tasks [FIXED JENKINS-27476]
- Remove any requirement from Jenkins Core to lock on the Queue when rendering the Jenkins UI [FIXED-JENKINS-27566]

My plan is to remove this in a while.

(cherry picked from commit e405078e)

(cherry picked from commit 4aad749c)

Conflicts:
	core/src/main/java/hudson/matrix/MatrixBuild.java
	core/src/main/java/hudson/matrix/MatrixConfiguration.java
	core/src/main/java/hudson/matrix/MatrixProject.java
	core/src/main/resources/hudson/matrix/MatrixProject/configure-entries.jelly

These users can still be instantiated, as would happen if there is no security and an anonymous user triggers a build -- the anonymous user would correctly be created and added to the User list. This fix merely prevents the saving of that user, and therefore prevents them from logging in.

There may be some plugins which trigger a build as the SYSTEM user, and that is not prohibited here.

Also prevent full names of 'anonymous', 'system' or 'uknown'. As discussed on SECURITY-166 this may encumber auditing since full names are used in most places in the UI

In 20340e18 @daniel-beck suggested updating this text.

* Save CM editor when it loses focus to ensure that test is saved back to textarea
* Turn on again CM editor for shell scripts

Also moving this switch to security settings.

Attempt to do it in pure CSS ala 3e9cdcbc failed so using CSS and JS combo.

(cherry picked from commit df9c92e4)

@kohsuke prefers that the option to disable signature checks not even be presented unless -Dhudson.model.DownloadService.noSignatureCheck is used.
(cherry picked from commit 54c85007)

(cherry picked from commit 1ac77750)

Conflicts:
	core/src/main/java/hudson/model/DownloadService.java
	core/src/main/java/hudson/model/UpdateSite.java

This expects that any mechanism that keeps builds handles their disappearance gracefully.

- Recorded the fix in changelog.
- Added a separate screen to show the unmigration instruction, which
  makes it easier for people to find it than search logs.

Was using JS to size panels. This change only effects screens > 768px. Less than 768px is responsive.

[FIXED JENKINS-23797] Produce human readable output in DiskSpaceMonitorDescriptor.DiskSpace.toString()

This makes the overall appearance of the page match what you would actually see from the console.
(Using: java -jar jenkins-cli.jar -s <URL> help <COMMAND>)
For some commands, such as SupportCommand (in support-core), this is critical,
since the expectation is that longDescription is a multiline fixed-width block.