Show username in sign-in notification emails

I got this email earlier:

This wasn't because I'd signed in to my regular account (smcgivern) from a new location. It was because I signed in to my admin account (smcgivern-admin) for the first time in a long time.

It took me a moment to realise this was what the email was for, because I could only tell that the email was for my admin account based on the email address. The two share an inbox.