[FIXED JENKINS-16397] Just displaying /asynchPeople constructed the expensive People object merely to decide whether or not to display the “REST API” link.

This change does not necessarily improve total performance, since the avatar is still computed.
But (1) the computation is correctly done in the work thread, not in the HTTP response thread;
(2) the computation is done just once for a given User, which could reduce load when many AJAX checks are done.

Plus fixing the wrong apply button handling in various code.

(Its index.jelly does assume getDisplayName is available.)

From what I was told by Emanuele today in the office hours, this came up
during the Copenhagen hackathon.

Update transient action after config submit, as trantient action may change due to change of view properties.

See https://github.com/jenkinsci/jenkins/pull/474 for the original conversation

The trick for backward compatibility is in the default implementation
that grants View.READ to those who have access to items.

than the default view - In the view object appended the system message
to the view description so that the system message now displays in all
views.

Added ViewGroup.getItemGroup() so that views can be used with ItemGroups in general, not just with Hudson.

Just import changes, from hudson.model.Jenkins -> jenkins.model.Jenkins,
and Javadoc comments.
Signed-off-by: NNigel Magnay <nigel.magnay@gmail.com>

Signed-off-by: NNigel Magnay <nigel.magnay@gmail.com>