Read only for branch for deployment (BB-7284)

Hi,

The currently deployment key is read only for the entire repository. It would be better if we can define it for specific branches of the repository.

I typically have a "release" branch that I want this deployment key to have access to. It should not have access to my "master", "experimental", etc branches.

This is especially useful when we use deployment tools such as Capistrano to deploy our code to a live production environment but allow us to restrict read only access to only a specific branch instead of the entire repo.