Remove usage of HstRequest/HstResponse/HstSite in HstLinkTag, HstLinkImpl and replace it by HstRequestContext and SiteMount

To be able to use <hst:link> without having a HstRequest, we need to remove the HstRequest logic from it. Everything should be possible with the HstRequestContext.

Similar, for REST environments that work on SiteMount's that do not have an HstSite associated, we should depend on SiteMount instead of HstSite