WebJars URLs should target the current wiki by default + support relative paths

With XWIKI-12878 we can specify the wiki in the WebJar URL. The problem is that loading a resource from a WebJar installed on the current wiki is the most common use case which means that now we have to write:

$services.webjars.url('org.xwiki.contrib:application-ckeditor-webjar', 'ckeditor.js', {'wiki': $xcontext.database})

all the time if you want to make sure your code works on a sub-wiki too. I think the webjar script service should do this by default, and we should make sure that

/xwiki/webjars/application-ckeditor-webjar/1.4/ckeditor.js?wiki=chess

fallsback on the main wiki and then on the farm if the WebJar is not installed on the sub-wiki.