Move a bunch of stuff to the workbench image.

- Move Bash completion of tools to the image (it makes sense to have
  them together).
- bfg download and script.
- Remove the binaries Makefile, remove almost all of the vendored files,
  merge everything back to a single Makefile.
- Delete the Python requirements.txt, it's all in the image anyway.
- Some nicer output in the Git post-merge hook.
- RabbitMQ admin script.
- Update the README.