Allow page_header events to prevent the sending of HTTP headers

In the includes/functions.php file, function page_header(), after the core.page_header_after event, the script sends several HTTP headers as seen here:

https://github.com/phpbb/phpbb/blob/develop-ascraeus/phpBB/includes/functions.php#L5050

Integrators may not want this to be done as they may have set their own or already started output (would generate a notice). The suggestion is to wrap the sending of headers in an if block so that the core.page_header_after event can set a variable that disables them.

A similar issue exists in the ACP in the function acp_page_header():
https://github.com/phpbb/phpbb/blob/develop-ascraeus/phpBB/includes/functions_acp.php#L111

It's worth noting that there is no core.adm_page_header_after event like there is in non-ACP function. Perhaps that should be added as well.