added server list rules agreement system closes #54

[JasonTable]

This adds an agreement system kinda similar to minecrafts eula=true. but this is for server list announcing. this will require some changes to minetest itself to add a server announce_tos_agree bool property to minetest.conf that makes it send "tos_agree":true in the json message. when the rules have not been agreed to, a 403 is returned with the rules in the body so that they show up in the server console.