Add collaborators to a draft security advisory with the REST API

At GitHub, we store multiple copies of every Git repository our customers push up. Every once in a while, one copy of a Git repository can wind up in a broken state. Usually, our maintenance processes fix the copy automatically, and no one ever notices the problem. Under very rare circumstances, the automation is unable to fix the problem, requiring manual intervention. To prevent further damage from accruing, the automation marks the repository "broken" and doesn't allow further changes (pushes or maintenance).

The GitHub web interface has long shown an informative message informing viewers that the repository is broken. With this change, the Git client will also receive a similar message. Previously, Git would report an unactionable, confusing, and very low-level error such as "fatal: bad tree object 3c8f2e6c8252929ce8334d52bd33b2bc358e7e4c".