REVIEW: [NXCM-5233] Introduce property to enable/disable session timeout from UI 

https://issues.sonatype.org/browse/NXCM-5233

Disables automatic pinging of Nexus in case that system property "nexus.ui.keepAlive" is set to false (defaults to true).

UI will call into "/service/siesta/wonderland/settings" to retrieve this setting before calling into nexus status.
The new REST resource returns a list of key/value pairs so we can extend this in the future.