Page picker in the "Template to use" field should be global

This improvement is based on https://forum.xwiki.org/t/how-to-use-one-template-for-several-subwikis/13470

If you create a template in sub wiki A and you want to use pages from another sub wiki B as the template pages, the search in the UI doesn't show pages from sub wiki B, only from the current sub wiki.

Better: show results from all wikis, or give you the tree to choose the page or something similar.