Don't assign new reviewers if there are already matching reviewers

[uesteibar]

It should handle multiple groups, for example:

group X => users A, B and C
group Y => users B, D and E

If when opening the PR, user B is already a reviewer, it shouldn't assign anyone else from team X, but it should still assign someone else from team Y.

Group priority is from top to bottom in the config file

[uesteibar]

I think we actually don't want this, so closing it