Update mailmap

The .mailmap file is used for generating stats about committers that we find useful.

## Verification

- [x] `git log --format="%aN <%aE>" sprint-F00...HEAD | sort | uniq` should produce a list of people with commits without any obvious duplicates among the people listed in the `.mailmap` file.
- [x] Rapid7 employees should always be referenced by their Rapid7 GitHub username.