Use Slack attachment for prettier issue post

Created by: felixfbecker

Slack doesn't use Slack apps (like the GitHub Slack integration) to unfurl links sent by other Slack apps/webhooks, so the unfurled link didn't contain issue details. But we have all the issue info, so this uses some manual Slack attachment to recreate it (or actually better, because we can include our project fields like "Size" and "Importance" as key/value lists which GitHub wouldn't inline)