Use bugbug to classify issues that are coming to web-bugs-private

[ksy36]

Once mozilla/bugbug#2330 is deployed we will be able to make a request to bugbug http service for issues classification.

As a first iteration, lets classify issues that are coming to our private repository and add a label. Once we confirm that it's marking most of the issues correctly, we could close the labelled issues as invalid (after they're moderated and moved to the public repository) and turn off the existing bot.

Existing implementation: https://github.com/mozilla/webcompat-ml-deploy/blob/master/docker/needsdiagnosis/run.py#L107

GET request to get results from bugbug http service is https://bugbug.herokuapp.com/needsdiagnosis/predict/github/webcompat/web-bugs/<issue_id>