Verify Developer Certificate of Origin (DCO) on each commit

When committing code, users need to run:

```
git commit -s
```

which will add a line similar to the following at the end of the commit message:

```
Signed-off-by: Jane Doe
```

Developers who don't want to remember to add `-s` every time they commit code can simplify it for themselves [via an alias for `git commit`][alias].

This check should ensure that:

* this line exists in each commit
* the name and email on this line matches the name + email of the commit itself

[dco]: https://developercertificate.org/
[alias]: https://github.com/JanusGraph/janusgraph/blob/2ffcb3bb690df2772693eb60d23a01976ae2a9f8/CONTRIBUTING.md#commit-changes-and-sign-the-developer-certificate-of-origin