Add github status integration

This adds Github status integration from the development branch on Buildbot Nine. Documentation for the feature can be found [here](https://github.com/buildbot/buildbot/blob/nine/master/docs/manual/cfg-statustargets.rst#githubstatus). Once there is a dedicated slave for checking pull requests (as mentioned in [ticket #2052](https://tahoe-lafs.org/trac/tahoe-lafs/ticket/2052#comment:13)), add a github token to the secrets file, and add the properties `github_repo_owner` and `github_repo_name` to the bot that will watch the testing branch. When commits are merged with the branch, their status should be updated on the pull request.