Make hst:showContextPath behavior consistent in the Page Model API

Currently, this option removes the context path from all the links, even external and resource ones. That produces invalid links and breaks SPA implementation in v14.1.

Next to it, this option does not take any effect in the preview (which is should for the Page Model API output). This causes the custom preview configuration on the SPA SDK side.

Requirements:

• resource API and binary links should be affected by the option hst:showcontextpath. If it is 'false', context path won't be in the links. If an end project doesn't want this, just don't use hst:showcontextpath = false on the page model api mount
• internal links have become relative to sitemap items in a different improvment so work correctly wrt contextpath
• the context path should be consistent for the Page Model API in both live and preview: either present in both or missing in both, depending on the hst:showcontextpath config