accept 'menu_order' for 'orderby' get_sites() argument

[gnowland]

Per core trac ticket 43770 https://core.trac.wordpress.org/ticket/43770#ticket it would be suggested that this plugin handle modifying the get_sites() function to allow 'orderby' to accept 'menu_order' and return sites in order of menu_order as set by this plugin.

[timohubois]

@gnowland it would be really nice if you can create a pull request for this.