mw: Implement support for ConfirmEdit and FancyCaptcha

* 'captcha' property from ConfirmEdit API is already exposed
  in ApiEdit and ApiVisualEditor through the 'edit' property
  in our response data.
* Add parameters 'captchaid' and 'captchaword' to ApiVisualEditor
  and mw.ViewPageTarget#getSaveOptions. ApiVisualEditor will
  forward these to ApiEdit which forwards them to FancyCaptcha.
* We display the captcha through a saveDialog warning.

Bug: 50356
Change-Id: Ia7d2102cba89d00ec8508e846061023b330ece4f
Merge "mw: Implement support for ConfirmEdit and FancyCaptcha"