[wp-forums] Support forum registration now has a re-captcha on it

[Otto]

In an effort to combat the rising tide of spam made by fake user
accounts, I've added a ReCAPTCHA to the /support forum registration
page. This should at least slow it down for a while, and perhaps
reduce the number of false user registrations we've been seeing (500+
per hour).

I'll be monitoring it for problems. Let me know if you spot any issues.

-Otto