Optionally make incoming data from Pull Requests private from users who don't have access to the source repository (BB-7213)

A while ago, a change was made to allow all the administrators and writers of a repository to view incoming data from a Pull Request. This change enabled team leads and such to have access to the incoming data, while not needing direct access to the source repository.

Recent requests have been made to change this behavior back for users who are using Bitbucket for university courses, so that other students cannot see each other's work being posted back to a master repository.

Having the ability to make this optional for a specific source repository may be a way to enable both practices to work.