Mandatory reviewers without default reviewers

Issue Summary

Today it is only possible to mandate approvals from default reviewers, who by definition all get added to all Pull requests.

While we do want to maintain a list of users, from which N become mandatory on the Pull requests, we don't want all of these users to be added to all Pull requests and start getting notifications. 

Steps to Reproduce

1. Add a list of users to Default Reviewers
2. Add a branch restriction to require N approvals from default reviewers

Actual Results

• Every default reviewer is added to every Pull request

  Expected Results

We would recommend the following design:

• Rename default reviewers to core reviewers.
• After specifying default reviewer list, add a checkbox to indicate a preference: "Add core reviewers to all Pull requests by default"
  • If this checkbox is ticked, all core reviewers get added to all Pull requests (like today)
  • If this checkbox is not ticked, this is a list of core reviewers the application is aware of but doesn't add to all Pull requests
• After specifying branch permission to mandate N approvals from core reviewers, the list of core reviewers comes to play
  • This list can be somehow exposed on the UI for endusers to see (since it's the Admin who created the list) but not by adding all of them to each Pull request

Workaround

There is no workaround available today