Handle CAPTCHAs with different mime types different
ConfirmEdit supports different CAPTCHA modules, which return different
CAPTCHAs. MobileFrontend actually supports only images as CAPTCHAs.
This change adds support for text based CAPTCHAs, like text/plain
(SimpleCaptcha and QuestyCaptcha) and text/tex (MathCaptcha), as well
as image based CAPTCHAs, like image/png (FancyCaptcha and ReCaptcha) and
image/jpg (not used, yet).
- Fix a FIXME