Do not notify label watcher who is already participating

[aneeshusa]

Currently, highfive will cc a label watcher on an issue even if they have already commented or been mentioned. @mbrubeck has been triaging a ton of issues lately (which is awesome), but the aforementioned behavior means highfive is leaving unnecessary comments; see servo/servo#11167 for an example where I am already participating in the issue.

This will require changes to APIProvider and GithubAPIProvider in newpr.py, as well as TestAPIProvider in test.py, to add a method to get the list of participants on an issue.

Files:

• newpr.py
• handlers/label_watchers/__init__.py
• Add a test to handlers/label_watchers/tests

[jdm]

Is there a github API that returns the current participants?