Using commit search to identify new contributors

[davidalber]

The current approach to checking for new contributors does not work in repositories with more than 500 contributors because the contributors list API endpoint only associates the first 500 committer emails with GitHub usernames. See #41 for more explanation.

This PR modifies the new contributor check to use the commit search API endpoint. A drawback of this approach is that (at least currently) that endpoint does not provide useful information for fork repositories. To address that, is_new_contributor returns False when the repository is a fork.

To summarize, here are pros and cons:

• [pro] Repositories with more than 500 contributors (e.g., rust-lang/rust) will no longer treat every PR from repeat contributors, beyond the first 500 contributors, like they are new contributors.
• [con] Nobody in a fork repository will be treated like a new contributor. I don't think Highfive is currently configured to support any fork repositories, so this won't be noticeable for now. I've asked a question here to see if this is a bug or can be worked around.
• [con] The Commit Search API is a preview that can change at any time, so there's a risk that this will break one day or just go away altogether. I think this is a better option than what Highfive is currently doing, and if you agree, I am going to follow-up on the PR by adding integration tests so that we will notice through automation if the API changes.

This conflicts with #117, so I'll need to rebase this or that, if one of them is merged. I wanted to get this out now to get commenting going.

[nrc]

I think this is a better approach - not working on forks seems like a theoretical problem at this stage