pages: browser code, first draft: - code for browser can now be specified if a page subdirectory named `_browser` exists - the default engine used is `Brython` (not yet implemented, so it will do nothing for now) - build_path now uses C.SITE_NAME_DEFAULT for default site instead of empty string, to avoid collision with other sites - ProtectedFile transtype path (first positional argument) to str, so a pathlib.Path can be used