Post ML replies back to GitHub issues as comments

[anssiko]

IIRC this feature was discussed earlier, but I couldn't find an existing issue so I opened a new one. Source for the request: https://lists.w3.org/Archives/Public/public-device-apis/2016Feb/0030.html

Implementation considerations: each message has a unique Message-ID to correlate replies with GH issues, but I believe some email clients do not retain this information in replies. To workaround this could try to encode the information in From, or include the ID in the message body.