Assign groups to review pull requests

Allow a group to be assigned to review a pull request.

The idea being that only one member of that group needs to approve the PR, but that the PR creator does not need to know who exactly will do the review.

Use Case Example:
Developers always needs approval from a database administrator (dba team) for any SQL files they modify in their source code.

Further Enhancements (possibly in later releases than the initial implementation):
When a group is assigned as a reviewer, a member of that group could choose to 'own' the reviewal to indicate to other group members that someone is already in the process of actively reviewing the PR.

Allow the PR creator to specify a certain number of members needing to approve the PR, or even the entire group.