The directory name (with full path). We make several assuptions:
1) dir/*.R has one answer, or we
take index.R, or we take first alphabetically; 2)
app_name is basename(dir_name); 3)
brew/*rhtml is empty (fullscreen), has one value, or has
index.rhtml or we take first alphabetically; 4)
authenticator (XXX to be determined, for now calls
make_session_manager)
...
passed to load_dir reference class method of
R_http instance.