Add ability to search in all allowed subwikis when searching from a subwiki

Right now when searching in a subwiki, only documents located in this subwiki are returned.

When on a farm like xwiki.org, it would be interesting to also be able to seach in the full farm from any subwiki. I would venture that if the subwiki is allowing global users (ie not a local, isolated subwiki) then it would be interesting to have a button/toggle to be able to search in all wikis that the user has access to (same rule as the Wiki Index), both for search suggest and main search.

Note that this would also allow us to drop our custom Search page in the xwikiorg flavor and allow us to have search suggest in subwikis on xwiki.org