Implement sync of GitHub pull request reviewers and Collaborator reviewers

[AlexeyKryuchkov]

Submitted on behalf of Debdas Mondal (HSBC).

GitHub pull requests may have reviewers assigned to them. It should be possible to synchronize the list of pull request reviewers and the list of a linked Collaborator review's participants.

[mtalexan]

Ditto for GitLab's Approvers List.

[mtalexan]

In general the GitHub/GitLab integration needs to be more granular in my opinion, and options are needed for what to pull across from a merge/pull request triggering a Collaborator review to be created.  This would be an extremely useful option that could be independently selected on a per-project basis.

What isn't clear is the role these Approvers should be given.  GitHub/GitLab doesn't have roles for Approvers, while they're critical in Collaborator.  The roles available are different between templates in Collaborator, so what role should these approvers be given?