Switch away from passing tokens in URL / support fine grained permission tokens

Right now we do this
```
# construct GitHub URL for pushing
REMOTE="https://$GITHUB_USER:$TOKEN@github.com/$REPO"

# Make sure the branch is on the server
notice "Pushing your branch to GitHub"
git push "$REMOTE" "$BRANCH"
```

This was handy in the past, but GitHub is introducing fine grained permission tokens, which apparently don't support this. It would be good to support these too