Prevent installing a XAR extension that requires programming rights if the current user doesn't have them

Idea: introduce some maven properties for EM so that XAR extension authors can indicate if an extension requires PR + have EM check that info before installing a XAR extension to prevent the user finding himself/herself in the situation where the installed extension doesn't work.

See https://forum.xwiki.org/t/unknown-macro-plantuml/642/9 for an example.