Add the ability to create an application in a nested space

Currently, it creates one space per application. In the future, we might want to create an application in a subspace.

Example of code that will cause a problem:

#set($templateRef = $services.model.createDocumentReference($doc.wiki, $doc.space, "${className}Template"))

(in https://github.com/xwiki/xwiki-platform/blob/master/xwiki-platform-core/xwiki-platform-appwithinminutes/xwiki-platform-appwithinminutes-ui/src/main/resources/AppWithinMinutes/ClassEditSheet.xml#L225-225)

must be replaced by:

 $services.model.resolveDocument("${className}Template")

ie: the reference will be created in the current (sub) space.