Allow registration of user id to automatically update discord rank

[Endaris]

Ported from the PA and ShockBot issues and suggestion sheet.

ShockBot should offer the option to register your PA id with it, ideally using DMs in order to protect your id.
That way users could register their id on the discord to always have access to the id when playing from other devices and it also opens up various integration options with the api of the panel attack server like assigning the correct discord rank role based on the player's ingame rank.