Create Discord plugin about gateway

[gigxz]

@shaunagm I already have a working Discord plugin on a branch so I'll create a PR to merge it in, even though PolciyKit won't use it (yet)

[gigxz]

Because the PK Discord integrator uses websockets to listen for actions so it can "govern" them, while this Discord Plugin just uses the interaction api, so it can't "listen" to events on platform-- it can only respond to users interacting directly with the bot.

The PK integrator doesn't really work, tohugh, because the socket is not deployed correctly so it just closes after some time. But there was hesitation to switch over to this gateway discord plugin because it would be losing functionality (you can no longer "govern" message posts, since you cant listen for all messages)

[shaunagm]

Sounds good. Remind me why PK won't use it?

[gigxz]

Closed with #168

Follow-up to implement identity linking, #171