Update by webhook

[KiaraGrouwstra]

for tracking purposes: it would be nice if we could update by webhook as well :)

[devzero]

I've been looking into this some more. I think what I'd like to try is making a simple server that sits somewhere accessible that listens for repo webhook calls, and also offers up a Server-Sent-Event URL that comin can query. What this would allow is long polling, which is much simpler than WebSockets or MQTT, but would also allow comin to update almost instantly without constantly polling the git repo and wouldn't require the endpoints to listen on anything. The code changes to comin could be minimal