Add supportsNS variable to templates/xwikivars.vm

Since the Nested Spaces feature has been implemented in XWiki, some contrib applications can be improved. An example is the Forum App where the Forums_MyForum/Topic hierarchy became Forums/MyForum/Topic.

In order to be able to handle both Pre NS and NS cases a variable could be defined in templates/xwikivars.vm like:
Before XWiki 8.0M2:

#set ($supportsNS = $services.extension.core.getCoreExtension('org.xwiki.platform:xwiki-platform-model').id.version.compareTo($services.extension.parseVersion('7.2')) >= 0)

With XWiki 8.0M2+:

#set ($supportsNS = $services.extension.core.getCoreExtension('org.xwiki.platform:xwiki-platform-model').id.version.compareTo('7.2') >= 0)

This is related to https://www.xwiki.org/xwiki/bin/view/FAQ/How+can+my+Extension+know+if+XWiki+supports+Nested+Spaces