Fix error on contact page where site key is undefined

[splennon]

In Contact.js there is an attribute called captcha_key in event, but line 26 of the js is referring to it as captcha_sitekey. Thus I confirmed that it is undefined in the browser rendered html.

This fix refers to the correct captcha_key and captcha on the contact page works now, where (I think) it could not have worked before.

My knowledge of this codebase is limited so you know what they say about advice :)