move and remove some magit extensions

Unfortunately @sigma's extensions to Magit have bit rotten a bit and it seems he hasn't the time to fix them right now. Since I have been contacted by email about these issues a few times now, I would like to do something about it.

I have adapted his `magit-tramp` and `org-magit` to contemporary Magit and performed some rudimentary testing. Because Yann currently doesn't respond to pull requests I have pushed them to the forks inside the `magit` organisation. The official upstreams are under his user account `sigma`. I have also contacted Yann, suggesting that the forks belonging to the `magit` organisation should become the official repositories. He has not responded to that suggestion yet, so the next best thing is for Melpa to fetch from the forks instead. The alternative would be to remove these packages because his versions are unusable with current Magit. Once Yann returns he will either declare the forks the official repositories or we can go back to his repositories once he has fixed the issues.

For his `magit-gh-pulls` there is no quick fix. I tried to do it but once it became clear that doing so would result in a complete rewrite I gave up. So I am removing the recipe for this package.

Then I also remove the recipe for `magit-commit-training-wheels` (not by Yann). It advices a function from `magit-log-edit` (without declaring a dependency on `magit` or `magit-log-edit`). Magit doesn't use `magit-log-edit` anymore, it used `git-commit-mode` which happens to contain similar (but better) functionality to that provided by the training wheels, and even references the same blog post as inspiration. The old library `magit-log-edit` is still available as an alternative but it is in "I only touch it if someone reports breakage"-mode, and soon I will completely stop supporting it.