Add a filter to array of allow ported number in multisite.

[spacedmonkey]

The allowed port numbers to setup a multisite are fixed. See this line.

        if ( ( false !== $has_ports && ! in_array( $has_ports, array( ':80', ':443' ), true ) ) ) {

However, a site maintainer may want to change these values to help enable serving their multisite from a custom port.

This ticket is a breakout from #21077