Add a Github reporter to the bot

[vrigal]

Depends #3254

The reporter publishes a review with the issues related to the diff, and approves if there are no issue.

[vrigal]

I was able to run the publication from a real Taskcluster decision task trigerred from github: https://firefox-ci-tc.services.mozilla.com/tasks/TbyUh4mPQb-vlvzXOR9Vrw.

I published the review to a custom repository : vrigal/ff-test#2.

The app credentials have a R/W access to pull requests with, with the following secret:

reporter: github
      client_id: <app_client_id>
      private_key_pem: |-
        -----BEGIN RSA PRIVATE KEY-----
        <secret_rsq_key>
        -----END RSA PRIVATE KEY-----
      installation_id: <integer_id>