Proposal for performance priority labels

Current description

To help clarify priority of issues, from the perspective of their impact on availability and performance of GitLab.com.

Closes https://gitlab.com/gitlab-com/infrastructure/issues/1943

Original description

I do not think this is a priority right now, but it stems from https://gitlab.com/gitlab-com/www-gitlab-com/merge_requests/5373/diffs which I broke into smaller merge requests.