Multisite networks

Works on a single site and on a WordPress Multisite network. Activate it on one site, or Network Activate it to switch it on across the whole network. Each site keeps its own redirects, its own broken links list and its own settings, and a site added to the network later is set up on its own. Who can manage redirects is chosen site by site too. A site deleted from the network takes both its redirects and its broken links list with it. A site deleted while the plugin was switched on for that site alone leaves those tables behind at the time, and deleting the plugin clears the leftovers away.