Added recaptcha2 plugin support

[TomRichter]

Added recaptcha2 plugin support so that using preregister and recaptcha2 in tandem works correctly. Without this change, recaptcha2's CAPTCHA is ignored during registration and preregister's own CAPTCHA may appear alongside it, depending on settings.

I also attempted to update the localized admin panel strings to acknowledge recaptcha2 as a valid option, but I don't speak any of these languages and couldn't make a educated guess for Japanese. It might be worth getting an actual speaker to proofread them, if possible.

The previous pull request introduced a bug that caused the original captcha plugin to not work correctly, resulting in builtin always being substituted in. This version does not have that problem.

[turnermm]

It will take a little while for me to get to this. In the meantime I am adding a link on the preregister plugin page to your update for users of recaptcha2:
https://www.dokuwiki.org/plugin:preregister#recaptcha2
This way it will, hopefully, get used and tested, and I will have a chance to familiarize myself with recaptcha2.

[TomRichter]

Thanks for adding the link. I'm using this branch on my own wiki, so I'll hopefully catch any remaining bugs or weirdness. Let me know if you have any questions.