Add a command to subscribe to landing notifications

[therottenshadow]

This has a problem, the way to do it would be to add a table in our database, each row containing discord user id, torn api key, time of arrival, destination and their wanted notification time, and have a while loop that goes through the table, if we are past the notification time, send a private message.

However, when the user is in Torn, unless we poll very frequently (10-15 minutes) there is no way to get notified that the user has left, so by polling frequently we are generating stress both on our server and on Torn's API servers.