Move template header inside form so CAPTCHA works

Also wrap it inside a section tag so the form's styling of gray labels
and full-width input fields doesn't apply to the CAPTCHA.  The same fix
is in the new Create account form (though that uses JavaScript to
reposition the FancyCaptcha image).

CSS gives the section a small bottom margin to space it from the form
contents, small enough to not affect form layout if there is nothing
in the template header.

Bug: 47371
Change-Id: I1523fe6551585ba731b0ca4207e22d614d0a46d8