Allow users to configure a "Pull Request Reviewer" role on the account level.

In situations that involve a huge number of repositories and users, it can be a big hassle to customize and keep track of the repository access by constantly changing them for the sake of allowing users to review the pull requests.

We should be able to configure a “Pull Request Reviewer” role where users who are added to this role can directly view all pull requests within the account.

Another alternative would be the functionality to configure the pull requests so that it can be viewed and merged by particular users without granting the repository access to them.