Reverting #1154 (superseding #1159).
There seems to be no way to export this over the REST API without incurring serious performance penalties.
Updating tests to fix more regressions
User.impersonate now requires that the user be a valid user.

These tests weren't using a SecurityRealm that recognizes these user names
Merge branch 'deconstructing-AbstractProject'
[FIXED JENKINS-22131] Merged deconstructing-AbstractProject branch from #1146.
Fixing a test failure.

The intent of this test is to make sure that a floating User object without any storage returns false from canDelete() method.
The earlier change made User.impersonate() to actually check if the user is a valid, so to make this test case work where it does
"user.impersonate" and "user2.impersonate2", I'm creating valid accounts for these two users.

Now what that means is that user2 is no longer a floating flyweight storage-less User, so it breaks the assumption of the
"User should not be able to delete because he is not saved." assertion.

To make this test case work, I'm introducing the 3rd user that is storage-less, and using it for the test.
Clarifying assertions that they are trying to test.
[JENKINS-21932] Added tests to reproduce JENKINS-21932, Future#get does not abort even when a task in the queue is canceled.
[JENKINS-21932] Make the slave used in the test offline explicitly.
[FIXED JENKINS-21932] Call Item#cancel when a task in a queue is cancelled.
Update sidepanel_de.properties
[maven-release-plugin] prepare release jenkins-1.555
[maven-release-plugin] prepare for next development iteration
updated changelog for release
updated changelog as a part of the release
merged back the RC branch
creating an RC branch
the trunk is toward 1.557-SNAPSHOT
Merge branch 'master' into DownloadSettings-JENKINS-19081
Removed DownloadSettings.checkSignature; back to using a plain system property for this.