Add hooks in API action=createaccount for Captcha

New hooks:
* AddNewAccountApiForm
* AddNewAccountApiResult

These hooks are used in ConfirmEdit here: Id628def

Sample API client: https://github.com/brion/api-createaccount

Bug: 46072

Change-Id: If5b7dab80ac85dbfa0f7a54a445356783df5e914
Add Status outparam for AbortNewAccount hook to fix API error handling

With this fix and relevant fix for ConfirmEdit in, an API account creation
attempt that fails the captcha check will return a much cleaner error such as:

{
  'error': {
    'code': 'captcha-createaccount-fail',
    'info': 'Incorrect or missing CAPTCHA.'
  }
}

Abort hooks that use the old interface and send a text message will now
be reported with the generic 'createaccount-hook-abort' message code, with the
string passed back intact.

Previously, the returned result would list the contents of the message
_as_ the error code, making it hard for a client to determine the error.

'AbortNewAccount' hook clients can add a '&$status=null' function paramater
on their signature, and along with the back-compat message parameter
you can set something like:

  $msg = wfMessage( 'captcha-createaccount-fail' )->text(); // back-compat
  $status = Status::newFatal( 'captcha-createaccount-fail' ); // new

This is done for ConfirmEdit in If9cc08e

Bug: 60008
Change-Id: I6ae34c00d1051d34363b6d654424be17dcb1ea30