Cannot create user with a '.' in the username

Created by: christinaforney

• Sourcegraph version: 3.6-rc.2

Steps to reproduce:

1. Go to site admin -> Users
2. Add new user
3. Set the user name to 'user.with.a.period' and email 'test@test.com'
4. Try to create an account

Expected behavior:

The user gets created

Actual behavior:

Form validation prevents me from saving the username containing '.'