campaigns: Handle GitHub merge events

Created by: ryanslade

GitHub does not send specific merge webhook events, instead it sends a PullRequest event with the close action and sets a merged flag.

https://developer.github.com/v3/activity/events/types/#pullrequestevent

When we see that flag set we'll persist it as a MergeEvent instead.

Closes: https://github.com/sourcegraph/sourcegraph/issues/10071