The script recaptcha_ajax.js contains minified non-free code which
we probably cannot redistribute. Since it talks to Google servers
anyway, it is better to just download it from Google directly instead
of shipping it.
This fixes#1932.
This is because otherwise it won't get picked up by python packaging.
This also fixes the problem where the "static" folder was found if
synapse wasn't started from that directory.
Callum Brown