Add trace support to the UI and API

[jonbodner]

The most common support question is: "I don't know why checks-out didn't approve my PR." So far, we haven't found any bugs in checks-out, but we have found that it is often unclear why checks-out makes the approvals that it does. It would be good to have a way for users to find out what checks-out current opinion of a PR is. This should be available via API and UI.

[cliffmeyers]

An API would be totally sufficient for our purposes, and I assume much easier to implement than a UI. I would suggest the following:

http://{checksout-domain}/api/repos/{orgname}/{reponame}/PR/{prnumber}

This would dump the various actions taken by checks-out against a PR, almost like an audit log, but just an array of simple objects.

I don't have any insight as to whether CO persists any of the actions it takes right now (either via a logging framework, or some other persistence engine). If it doesn't, this would obviously be a good chunk of work. However if the information already exists, then this shouldn't be too difficult?