Dots in usernames rejected in user settings frontend

Created by: dadlerj

• Sourcegraph version: SGdev

In https://github.com/sourcegraph/sourcegraph/pull/4690, we made it possible to create usernames from SSO signups that include dots. It's still not possible to change your username to include a dot in the UI though.

Steps to reproduce:

1. Visit your User settings > Profile
2. Try to change your username to add a dot
3. See the error below