Add Discord as notification service

[leumasme]

Discord has two obvious ways of sending notifications:

• via Webhooks
  • +Simple HTTP Post request to an easily created URL (via channel settings)
  • -Can only send to Channels in Discord Servers, not DMs
• via a Bot
  • +Allows sending messages via DMs
  • -Slightly Harder setup (creating an application with a bot on the Discord Developers page)
  • -Complex (websocket) protocol, effectively requires a Library

[Fluff663]

Between the two I would prefer to see webhooks implemented

[amirasyraf]

Not sure why not just add a generic webhook support so it could work with many other custom integrations