Instead, use a distance of 0.5x the width/height of the label.

This improves appearance in combination with JENKINS-22688 for
different font sizes.

Debian postinst: check for present user/group before adding them

pending was null as soon as task started running, so it was possible that another thread submits the task and have that executed while the first one was still running

This is in preparation of resolving dependencies within "List<URL> all"

Fix issue where restart would not properly display username if user was not loaded.

In git commit 7216aab0 the check for present user was removed
and therefore upgrading the Debian package on existing Jenkins
installations fails. Also the newly added code for adding a
configurable Jenkins group is missing a check for a possibly
already present Jenkins group.

Closes: JENKINS-22710

Excluded */empty.gif, which seem to be broken

Conflicts:
	war/src/main/webapp/images/title.png

This is a much simpler fix

Conflicts:
	core/pom.xml

Conflicts:
	core/src/main/java/hudson/model/UpdateCenter.java

- coding style was not matching the rest of Jenkins
- IOUtils.closeQuietly is a lot more concise way of achieving the same
  thing.
- AtomicFileWriter.abort() does close, so no need to close it one more
  time.

Queue length has to count individual SubTask, so that heavy weight jobs
correctly provision required number of slaves through Cloud.

Bringing this to the parity with ParameterizedJobMixIn.doBuild()

Instead of returning JSON, it returns "201 Created" and responds with the location of the queue item, through which the caller can track how it passes through the queue and eventually gets built/cancelled.

The approach in pull request #750 that relies on
AbstractProject.getQueueItem() is inherently unreliable, because
item can be immediately built before the JSON is rendered, or it might
corresponds to other queued items if the build supports concurrent
builds, etc.