renamed some classes and Twig functions to more descriptive names (refs #6871)

HttpContentRenderer has been renamed to FragmentHandler.
The render and render_* Twig functions have been renamed to fragment and
*_fragment.
The RendererStrategy subnamespace has been renamed to
Fragment.
The strategy classes now have Fragment in their names.
ProxyRouterListener has been renamed to FragmentListener
The router_proxy configuration entry has been renamed to fragments.