Add branch label to applicable types

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.