Add security headers to login/register pages

[jonathanstegall]

WordPress Core has this function:

/**
 * Send a HTTP header to limit rendering of pages to same origin iframes.
 *
 * @since 3.1.3
 *
 * @see https://developer.mozilla.org/en/the_x-frame-options_response_header
 */
function send_frame_options_header() {
	header( 'X-Frame-Options: SAMEORIGIN' );
}

We should use this on all pages generated by this plugin.

[jonathanstegall]

This plugin is currently run by using shortcodes that get loaded on pages. That might have to change, maybe to use the cortex routes instead. It'd be worth it though, I think.