Improve handling of complex issue description formatting

[cvitter]

Currently complex formatting (color, underline, strike through, etc) in the issues: description field are converted to plain ASCII characters using description.encode('ascii','ignore').strip() to make the content safe for posting in Mattermost.

Ideally there should be a way to convert some of this formatting to Mattermost markdown (https://docs.mattermost.com/help/messaging/formatting-text.html). Even better if we make it a reusable library...