[JENKINS-15757] Ignore 'remember me' if disabled in configuration

A new test for the existing 'Remember me' functionality that verifies that
the cookie is set, and a new test showing that, even if requested by the
user, no cookie will be set, if 'Remember me' is disabled in the security
configuration.

"Jenkins is loading" page gets set up before Jenkins instance gets
initialized.

these messages were purged in 772bd8f6
for being unused, but they are actually active.

[FIXED JENKINS-20191] Add Test button to check proxy connection

This change determines in the current items parent is the Jenkins
instance. If so, behavior is as before. Otherwise, link to parent
and use the label 'Up' instead of 'Back to Dashboard' similar to
Folders Plugin.

[FIXED JENKINS-20074] Do not report irrelevant exceptions thrown in the process of reporting a (possibly relevant) exception.
Merge branch 'uncaught-socket-exceptions-JENKINS-20074'

Conflicts:
	core/pom.xml

Fixed text alignment about icons on legend page

Distinctive icon for labels

Uses http://rrze-icon-set.berlios.de/tango/gallery.html#attribute

The previous implementation of 'disable remember me' just removes the
checkbox from the UI. Users still can set the parameter using a web
browser extension, or patch the login page using a Greasemonkey script
and keep using the feature. And just after the feature is enabled, users
still might be able to use it if they loaded the login page early.

This change prevents that by checking the current configuration and, if
'remember me' is disabled, ignoring any headers set.

[JENKINS-20280] Decide variable overriding order with topological sorting

In this case display the reported exceptions differently and be done with it.
java.lang.NullPointerException
	at hudson.diagnosis.OldDataMonitor.report(OldDataMonitor.java:178)
	at hudson.util.RobustReflectionConverter.doUnmarshal(RobustReflectionConverter.java:312)
	at hudson.util.RobustReflectionConverter.unmarshal(RobustReflectionConverter.java:222)
	at com.thoughtworks.xstream.core.TreeUnmarshaller.convert(TreeUnmarshaller.java:72)

[JENKINS-20280] Make it easy to change the implementation. Especially Map.Entry is not so convenient to handle.