Use new hooks in API action=createaccount for Captcha

Hooks used:
* AddNewAccountApiForm
* AddNewAccountApiResult

This adds a 'captcha' section to the results with the same format
as we provide for editing; you'll get this for instance at the
same time as you do a first-request that prompts for a token.

No modification to the result status is included; presence of
the 'captcha' section is assumed to be enough to prompt the client
to fetch and render the captcha prompt.

Failure to pass a captcha will return an API error message; a
subsequent commit fixes problems with that error message being
hard to machine-read.

Note that logic from inside Captcha::confirmUserCreate has been
pulled out to Captcha::needCreateAccountCaptcha so we don't
send captcha information to users who don't need it.

Requires core changes: If5b7dab8

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

Bug: 46072

Change-Id: Id628defaeab2bf5979ca8f4284d14fc42d9c3e46
(cherry picked from commit 78e6f5ec79e3f624ae90ae29a8359eb42001dfaf)