Allow underscores in usernames

[MichaelEischer]

What is the purpose of this change? What does it change?

Allow usernames containing underscore again, after they were forbidden by #131.

Was the change discussed in an issue or in the forum before?

Fixes #182

Checklist

• [x] I have enabled maintainer edits for this PR
• [ ] I have added tests for all changes in this PR
• [ ] I have added documentation for the changes (in the manual)
• [x] There's a new file in changelog/unreleased/ that describes the changes for our users (template here)
• [x] I have run gofmt on the code in all commits
• [x] All commit messages are formatted in the same style as the other commits in the repo
• [x] I'm done, this Pull Request is ready for review