Basic Implementation of new GitHub Pull–Requests

TODO:

- [ ] Unify dispatchers? I like the fact that GitHub and Trello events appear in different controllers, but having different dispatchers with our design probably isn't the best. Seems like there is a duplication of knowledge between GitHub.
- [ ] Pack together `BranchTracker`, `RepositoryFinder` and `TrelloCardUpdater` in `TrelloTracker` (maybe helps to solve the issue above)
- [ ] Adapt current GithubController to use one action only (thus, implement PushEvent handling)
- [ ] How should we deal with templates? Should `BranchTracker` be responsible for knowing which templates or comments to render?
- [ ] Finally remove useless route (`callback_v2`)