Reviewers cannot be assigned unless they are members of the github repo for the assessment

[millerh1]

Reviewers cannot be assigned to an assessment unless they are already members of the github org for that assessment.

This means a reviewer request will fail unless the reviewer has already been invited to the org as an owner & has accepted their invite already.

This means that the process of onboarding reviewers needs to involve them be invited to the org by the GH bot and then accepting it BEFORE we start assigning them to review things.