Add support for recaptcha during the create account flow

If the feature is enabled and recaptcha keys are given in config, then a recaptcha box is displayed in the UI when creating a user and a recaptcha response code *must* be sent with the create API call for it to succeed.

config.py

@@ -20,7 +20,7 @@ CLIENT_WHITELIST = ['SERVER_HOSTNAME', 'PREFERRED_URL_SCHEME', 'MIXPANEL_KEY',
                     'AUTHENTICATION_TYPE', 'REGISTRY_TITLE', 'REGISTRY_TITLE_SHORT',
                     'CONTACT_INFO', 'AVATAR_KIND', 'LOCAL_OAUTH_HANDLER', 'DOCUMENTATION_LOCATION',
                     'DOCUMENTATION_METADATA', 'SETUP_COMPLETE', 'DEBUG', 'MARKETO_MUNCHKIN_ID',
-                    'STATIC_SITE_BUCKET']
+                    'STATIC_SITE_BUCKET', 'RECAPTCHA_SITE_KEY']
 
 
 def frontend_visible_config(config_dict):
@@ -406,3 +406,8 @@ class DefaultConfig(object):
   # Example: 10 builds per minute is accomplished by setting ITEMS = 10, SECS = 60
   MAX_BUILD_QUEUE_RATE_ITEMS = -1
   MAX_BUILD_QUEUE_RATE_SECS = -1
+
+  # Site key and secret key for using recaptcha.
+  FEATURE_RECAPTCHA = False
+  RECAPTCHA_SITE_KEY = None
+  RECAPTCHA_SECRET_KEY = None