Add branch label to applicable types

[krancour]

It would be nice if subscribers could optionally narrow their subscriptions to applicable event type using a "branch" label. In this way, one could, for instance, express an interest only in events related to branch foo.

Great care needs to be taken in implementing this. The indicator that something is related in some way to a given branch will vary from one webhook type to the next. A pull request webhook illustrates this point. A pull request references both a source and target branch. While the source branch may be the right thing to put in the Git.Ref field (the thing to be "built"), the target branch is the right thing to put in the label.