#997 Work on Contact Page

Template modifications for contact page as per this:
https://github.com/mybb/mybb/issues/997

Modified templates: "post_captcha" & "contact".
No too sure here but could you use ```{$mybb->settings['bburl']}``` here on case the ```contact.php``` file is moved off the forum root?

I'm too sure so maybe I'm being just a little too picky.