Convert remaining gitweb links to gitlab ones

There are still some gitweb links in the README.md file which should get converted to gitlab links. It might not be worth it adding things to tidy as the conversion of some of the remaining gitweb links is not that straightforward (e.g. the webwml ones) and we therefore might be better off by just doing it manually.

However, Gitlab might not even have all the features needed yet and we might not have all needed repos at our Gitlab instance either right now. Thus, we might just make incremental progress at this point...