Improve logic in `Guard_Network::network_redirect()`

This prevents a loophole when redirecting to the network home url, while
the network home site is protected and not allowed for the current user.
Now the first allowed sub site is used as a redirection alternative.
Make the 'Guard' string translatable across the plugin
Consider network-only setting when filtering a user's sites
Shorten logic in `Guard_Network::sites_of_users()`
Remove the term 'Network' from the network settings sections titles
Refactor script loading and use it on the network settings page