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.

static/directives/signup-form.html

@@ -30,6 +30,10 @@
            match="newUser.password" required
            ng-pattern="/^.{8,}$/">
 
+    <div class="captcha" quay-require="['RECAPTCHA']"
+         ng-class="{'expanded': newUser.password == newUser.repeatPassword && newUser.password}">
+      <div vc-recaptcha ng-model="newUser.recaptcha_response" key="Config.RECAPTCHA_SITE_KEY"></div>
+    </div>
 
     <button id="signupButton"
             class="btn btn-primary btn-block landing-signup-button" ng-disabled="signupForm.$invalid" type="submit"
@@ -39,6 +43,7 @@
     </button>
   </form>
 
+
   <div class="cor-loader" ng-show="registering"></div>
   <div class="co-alert co-alert-info" ng-show="awaitingConfirmation && hideRegisteredMessage != 'true'">
     Thank you for registering! We have sent you an activation email.