Formalize Git Workflow

Desiderata:

- All commits should be signed by a maintainer
- All maintainer PGP keys should be public
- No commits should be signed by GitHub
- Tests should pass at each commit (i.e. we should avoid a sequence of commits with failing tests in the middle, even if they pass on the final commit in the sequence)

I found [github-merge.py](https://github.com/bitcoin-core/bitcoin-maintainer-tools/blob/master/github-merge.py), a script for merging commits into bitcoin-core, so we should probably just do whatever they do.