R1:14569ae4918e

Add a user-accessible hook for dumping debug code into an install

Summary:
Currently, there's no easy way for me to tell a user "run this code from the webserver and tell me what it says". Sometimes installs can add new .php files to, e.g., `webroot/rsrc/`, but this is setup-dependent and not universal. Generally I resort to saying "put this into index.php", but that's error prone and not acceptable on active installs.

Add a "debug" controller so I can instead say "put this into support/debug.php, then…