Post build notifications

[wraithan]

I think it'd be pretty rad to have a post build call back, so when a build of your docs complete it fires off a callback ala github post receive hooks. Probably only useful for notification bots but doesn't seem too hard to maintain either.

[ericholscher]

Might be nice to have an email callback as well, so people can get alerted to failures.

[wraithan]

Celery task to do the callback/email async.

Settings to selectively enable callback and email for each build type, default to HTML only

Send email and/or callback for each build type that is enabled.

[d0ugal]

+1 for failure alerts, i just came here to see if that idea was open. I just discovered I broke docs for a project a few days ago.

[wraithan]

Have a partial implementation (email and webhook) but doesn't have UI yet, working on that now.

[ericholscher]

Closing in reference to #258 -- which will track the work until it's done.