Enhancements to wp-signup.php and wp-activate.php

[geminorum]

While #17948 going on, I suggest calling get_header()/get_footer() along with 'singnup'/'activate' parameter on wp-signup.php and wp-activate.php.
Something like:

get_header( 'signup' );
get_footer( 'activate' );

Also adding before_activate_form and after_activate_form action hooks into wp-activate.php just like wp-signup.php