Allow commenting on GitHub pull requests

[danvk]

webdiff provides an alternative way to view GitHub PRs. But you're typically looking at a PR because you're reviewing it, so it feels incomplete without being able to publish comments.

This would be a convenient way to get my most-desired GitHub feature, draft comments w/ batch publishing. It would require a few additions:

• Clearer instructions for GitHub login.
• Local (on-disk?) storage of draft comments.
• A screen which displays all your draft comments.