Require github_api for RepoActivator

Fixes the following error when activating a repo:

> NameError (uninitialized constant RepoActivator::GithubApi):
>   app/services/repo_activator.rb:4:in `block in activate'
>   app/services/repo_activator.rb:22:in `change_repository_state_quietly'
>   app/services/repo_activator.rb:3:in `activate'
>   app/controllers/activations_controller.rb:10:in `create'
Comment on commits

Hound now complains about long subject and body lines in accordance with
<http://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html>.
Introduce CommitCommentingPolicy

As not to duplicate comments.
Fix broken spec
Fix NoMethodError: undefined method `map' for nil:NilClass
Fix bug trying to post empty comment
Fix bug: subject line can be empty