support discord webhooks

[intrand]

Would you please consider adding proper support for discord webhooks to fluxcloud?

Although you can configure fluxcloud to use a discord webhook using the /slack emulation trick (eg, https://discordapp.com/api/webhooks/1234567890/abc123efg456/slack) and it works for smaller flux repo changes, fluxcloud will choke sending very large payloads.

Here's the error I get when fluxcloud is sent a giant payload from flux:

Could not post to slack, status: 400 Bad Request
Exporter Slack got an error: Could not post to slack, status: 400

I assume that discord limits webhook payload sizes to a smaller size than slack or something like that. Either way, the /slack endpoint is not a reliable way to receive fluxcloud messages on discord at the moment.

Thanks!