Load extensions relative to CMS location (including context path if present)

The URL path of an extension is now used as-is to load the extension. That means that any value goes. However, since we'll use a resource servlet in the CMS to serve the extension for now, extensions should be loaded relative to the location of the CMS. For example, when the CMS runs at http://localhost:8080/cms, and the URL path of the extension is "/extensions/test", the extension should be loaded at http://localhost:8080/cms/extensions/test. And when the production CMS runs at https://cms.example.com, that same extension should be loaded at https://cms.example.com/extensions/test.