Allow multiple team-based PR reviews

While there is STASH-2870 to cover the fact that automatic assignment of reviewers to PRs is not part of Stash, and while the Workzone plugin allows teams to be automatically added, we'd really like to be able to require that PRs have two distinct reviews before a merge is approved - it should have peer-based developer approval, as well as approval from the QA team.

So, it would be excellent if Stash supported multiple "teams" of reviewers, and that for each team we could set the minimum number of reviewers required for that team to have successfully approved the PR, rather than just having everyone in one group and setting a higher level of approval.