[seL4] Pull requests now directly merged on GitHub

The next step in the making contributions easier is now complete:

Pull requests for repositories in the seL4 GitHub org can now be merged on GitHub directly, without the loop thought Data61's CI infrastructure.

Hopefully this will make merging pull requests faster and less painful.

The exception for now are repositories with manifests (such as sel4test-manifest), because parts of these manifests are generated automatically. We're working on moving these as well. The testing and verification regime is the same as before, including extensive hardware tests and proofs, but the results and logs should now be visible directly on GitHub.

Happy merging :-)

Cheers, Gerwin