Fixes #1163 Added re-captcha support to registration page.

Added re-captcha support to FH registration page.
Fixes #1163 

Implementation is a bit unusual since our registration is done though a form via the `registrations` django app.

Added ability to disable captcha requirement and set the settings (private key) on both fh.org and dev.fh.org.

@prabhasp it would be good to get the private key from the `local_settings.py` and put it in modilab's digital safe ?
@mberg it's deployed on dev.