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.

endpoints/api/user.py

@@ -2,6 +2,7 @@
 
 import logging
 import json
+import recaptcha2
 
 from flask import request, abort
 from flask_login import logout_user
@@ -183,6 +184,10 @@ class User(ApiResource):
           'type': 'string',
           'description': 'The optional invite code',
         },
+        'recaptcha_response': {
+          'type': 'string',
+          'description': 'The (may be disabled) recaptcha response code for verification',
+        },
       }
     },
     'UpdateUser': {
@@ -382,6 +387,19 @@ class User(ApiResource):
       abort(404)
 
     user_data = request.get_json()
+
+    # If recaptcha is enabled, then verify the user is a human.
+    if features.RECAPTCHA:
+      recaptcha_response = user_data.get('recaptcha_response', '')
+      result = recaptcha2.verify(app.config['RECAPTCHA_SECRET_KEY'],
+                                 recaptcha_response,
+                                 request.remote_addr)
+
+      if not result['success']:
+        return {
+          'message': 'Are you a bot? If not, please revalidate the captcha.'
+        }, 400
+
     invite_code = user_data.get('invite_code', '')
     existing_user = model.user.get_nonrobot_user(user_data['username'])
     if existing_user: