Support new Hooks API in existing hooks

We should let users write custom hooks using the new Hooks API in Bitbucket, whilst still supporting the old way of writing them.

Main advantage is that commits are streamed to you, so we don't have to support that ourselves. It's also easier to only concern yourself with one change, for example just changed added to the repository or when a branch is created.

This is particularly useful in the trusted commit authors hook where we want to have only the commits that don't exist anywhere else in the repository.