Integrate GitHubAlerter with factory classes

[dillu24]

The factory class implementations for alerting where done after the GitHubAlerter was implemented, therefore they were not integrated.

In this ticket we should do the following:

• [ ] Implement the GitHubAlerter similarly to other alerters by integrating it with the alerting factory.
• [ ] We must make sure that the GitHubAlertersManager has a similar implementation as the other managers.