ConfirmEdit, captcha version

This will now display the captcha to be solved in the error message.

Some caveats we may want to keep in the back of our heads:

1/ Biggest potential problem here is, upon successfully completing the submitted
post (with ConfirmEdit solution), the API will respond that the ConfirmEdit
params were unused.
This happens because those params aren't requested via the API's designated
getParameter methods (or even defined in the API's getAllowedParams).
ConfirmEdit just fetches them from the Request object.

Basically, this happens because I treat the ConfirmEdit implementation as not-
API. It has *some* (very limited) API stuff that I *could* use (if I modified
ConfirmEdit a bit). However, ConfirmEdit's API part doesn't render the form, so
that would be left up to us (= Flow), which is something I don't want to have to
do, since there are multiple Captcha types.

So basically, due to how this is implemented, the API will always respond with a
warning after succesfully submitting the correct captcha solution. That warning
won't affect anything, but might potentially be confusing to bot authors.

2/ Along with the captcha form HTML, some captcha implementations need some
CSS/JS. Those are extracted from $wgOut & responded along with the form HTML.
There's no arguing that this is not the best kind of programming, but since
ConfirmEdit is messy, has multiple different implementations & completely
outside of Flow, this currently seems to be the best solution. And it works ;)

Bug: 59835
Change-Id: I815136a108c3df38d69edcbb3ba4bfffae45d7e7