add contacts dir, leave mail files as is

- restructured storage.lua so reading/writing json is not duplicated
- when a player joins and has no contacts file yet, automatically add all players he wrote to

onjoin.lua

@@ -16,4 +16,6 @@ minetest.register_on_joinplayer(function(player)
 
 		end
 	end, player:get_player_name())
+
+	mail.migrate_contacts(player:get_player_name())
 end)