Support Discord as a first-class notification/alert target

[n1ru4l]

Currently, it is only possible to integrate notifications/alerts into Slack by using a webhook proxy. It might make sense to have a first-class integration for Discord.

[kmcgill88]

I have a need for Microsoft Teams. I'm happy to contribute; would you be open to PR's?

[kamilkisiela]

we're always open for PRs, yes.

[capaj]

@kmcgill88 PR for MS Teams is opened, should land soon